What is output voltage ripple rejection

Ripple Voltage in Rectifiers

Ripple Voltage As you have seen, the capacitor quickly charges at the beginning of a cycle and slowly discharges through RL after the positive peak of the input voltage (when the diode is reverse-biased). The variation in the capacitor voltage due to the charging and discharging is called the ripple voltage. Generally, ripple is undesirable; thus, the smaller the ripple, the better the filtering action, as illustrated in Below Figure.

Fig : Half-wave ripple voltage (blue line).

For a given input frequency, the output frequency of a full-wave rectifier is twice that of a half-wave rectifier, as illustrated in Figure 1. This makes a full-wave rectifier easier to filter because of the shorter time between peaks. When filtered, the full-wave rectified voltage has a smaller ripple than does a half-wave voltage for the same load resistance and capacitor values. The capacitor discharges less during the shorter interval between full-wave pulses, as shown in Figure 2.

Fig 1 : The period of a full-wave rectified voltage is half that of a half-wave rectified voltage. The output frequency of a full-wave rectifier is twice that of a half-wave rectifier.

Fig 2 : Comparison of ripple voltages for half-wave and full-wave rectified voltages with the same filter capacitor and load and derived from the same sinusoidal input voltage.

Ripple Factor

The ripple factor (r) is an indication of the effectiveness of the filter and is defined as

where Vr(pp) is the peak-to-peak ripple voltage and VDC is the dc (average) value of the filter’s output voltage, as illustrated in Below Figure. The lower the ripple factor, the better the filter. The ripple factor can be lowered by increasing the value of the filter capacitor or increasing the load resistance.

For a full-wave rectifier with a capacitor-input filter, approximations for the peak-to-peak ripple voltage, Vr(pp), and the dc value of the filter output voltage, VDC, are given in the following equations. The variable Vp(rect) is the unfiltered peak rectified voltage. Notice that if RL or C increases, the ripple voltage decreases and the dc voltage increases.

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Output Ripple Voltage

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60V step-down DC/DC converter maintains high efficiency

Output ripple voltage

The output ripple voltage for the circuit in Figure 75.1 , using a tantalum output capacitor, is approximately 35mV_P–P ( Figure 75.4 ). Peak-to-peak output ripple voltage is the sum of a triwave (created by peak-to-peak ripple current in the inductor times the ESR of the output capacitor) and a square wave (created by the parasitic inductance (ESL) of the output capacitor times ripple current slew rate). A significant reduction in output ripple voltage to 12mV_P–P can be achieved using a ceramic output capacitor ( Figure 75.4 ). With negligible ESR, the ceramic output capacitor reduces the portion of output ripple voltage generated by inductor ripple current times capacitor ESR. The useful feedback response zero provided by the tantalum output capacitor ESR for loop stabilization is now replaced by a capacitor inserted across R1 in the feedback resistor network.

Figure 75.4 . Output Ripple Voltage Comparison (Tantalum vs Ceramic Output Capacitor)

Dual output regulator uses only one inductor

Output ripple voltage

Output ripple voltage is determined by the ESR of the output capacitors. The capacitors shown are AVX type TPS surface mount solid tantalum which are specially constructed for low ESR ( _P–P, so an ESR of 0.1Ω in C1 will give 30mV_P–P output ripple. It is interesting to note that this ripple current is about one half of what would be expected for a buck converter. This occurs because the two windings are driven in parallel, so magnetizing current divides equally between the windings.

Ripple current peak-to-peak into the −5V output capacitor is approximately equal to twice the negative load current. The wave shape is roughly rectangular, and so is the resultant output ripple voltage. A 100mA negative load and 0.1Ω ESR output capacitor will have (2)(0.1A)(0.1Ω) = 20mV_P–P ripple. A word of caution, however; the current waveform contains fast edges, so the inductance of the output capacitor multiplied by the rate-of-rise of the current will generate very narrow spikes superimposed on the output ripple. With capacitor inductance of 5 nH, and dI/dt = 0.05A/ns, the spike amplitude will be 250mV! Now for the good news. The effective bandwidth of the spikes is all above 20MHz, so it is very easy to filter them out. In fact, the inductance of the output PC board traces (20 nH/in) coupled with load bypass capacitors will normally filter out the spikes. The only caveat is that if the load bypass capacitors are very low ESR types like ceramic, they should be paralleled with a larger tantalum capacitor to reduce the Q of the filter.

Both outputs can be shut down simultaneously by driving the LT1376 shutdown pin low. An undervoltage lockout function can also be implemented by connecting a resistor divider to the shutdown pin. See the LT1376 data sheet for details.

High efficiency, high density, PolyPhase converters for high current applications

Improved load transient response

The influences of PolyPhase techniques on the load transient performance are numerous. First, the reduced output ripple voltage allows more room for voltage variations during the load transient because the ripple voltage will consume a smaller portion of the total error budget. With the same number of capacitors on the output terminals of the power supply, the sum of the overshoot and undershoot can be reduced dramatically. Second, the reduced ripple current allows the use of lower value inductors. This speeds up the output current slew rate of the power supply. Consequently, PolyPhase helps improve the load transient performance of the power supply. Figure 14.6 shows the output voltages during a load transient. It is noted that the two circuits have the same electrical design. The dual-phase technique reduces the voltage variation from 69mV_P-P to 58mV_P-P, a 16% reduction with no changes in component values. The inductor values could be reduced while still achieving lower output ripple voltage than the single-phase design, and further improvement in the peak-to-peak voltage variations for the load transient response could be realized.

Figure 14.6 . Measured Output Voltage During Load Transients (V_IN = 12V, V₀ = 2V, f_s = 250kHz. Load Steps: 5A to 20A and 20A to 5A, 50μs Rise and Fall Times. Time Scale: 500μs/DIV)

Low cost, high efficiency 30A low profile PolyPhase converter

Wei Chen , Craig Varga , in Analog Circuit Design, Volume Three , 2015

Conclusion

PolyPhase converters using the LTC1629 reduce the size and cost of the capacitors and inductors due to input and output ripple current cancellation. Lower output ripple voltage and smaller inductors help improve the circuit’s dynamic performance during load transients. The LTC1629 helps minimize the external component count and simplifies the complete power supply design by integrating two PWM current mode controllers, true remote sensing, selectable phasing control, inherent current sharing capability, high current MOSFET drivers plus protection features (such as overvoltage protection, optional overcurrent latch-off and foldback current limiting) into one IC. The resulting manufacturing simplicity helps improve power supply reliability. High current MOSFET drivers allow the use of low R _DS(ON) MOSFETs to minimize the conduction losses for high current applications. Lower current ratings on the individual inductors and MOSFETs also make it possible to use low profile, surface mount components. Therefore, an LTC1629-based PolyPhase high current converter can achieve high efficiency, small size and low profile simultaneously. The savings on the input and output capacitors, inductors and heat sinks minimizes the overall cost and size of the complete power supply.

Versatile industrial power supply takes high voltage input and yields from eight 1A to two 4A outputs

Unique power control and features

The I 2 C interface allows extensive control of regulator operation. Each regulator may be set to a high efficiency Burst Mode operation to save power at light loads or set to forced continuous mode for lower output ripple voltage . Each regulator can also have the switching cycle phase shifted by 0°, 90°, 180°, or 270° with respect to the reference clock to allow a lower input ripple current when multiple outputs are supplying large loads. Another feature is the ability to manipulate each output voltage up or down by adjusting the feedback reference voltage from the default 725mV setting in 25mV steps (ranging from 425mV to 800mV). The I 2 C interface is also used for reporting error conditions for each regulator.

The LTC3375 has a reset ( RST ‾ ) pin and an interrupt request ( IRQ ‾ ) pin, which can be programmed to report when any regulator’s output voltage has dropped below 92.5% of the regulation point. The IRQ ‾ pin can also be programmed to report when the input voltage drops below the undervoltage lockout (UVLO) threshold or when the die temperature has reached a set temperature threshold. The regulator’s PGOOD and UVLO status, the die temperature warning and the measured die temperature can be monitored by the microprocessor via the I 2 C interface.

One problem with microprocessors is that a software bug can cause the program to hang. The LTC3375 includes a watchdog timer input (WDI) pin to monitor the SCL pin or some other pin to determine if the software is still running. If the software has stopped running, the watchdog timer output (WDO) pin can be used to reset the microprocessor or power down the HV buck and the LTC3375 buck regulators. Connecting the WDO pin to the RST ‾ pin of a microprocessor causes the microprocessor to reset when the WDT is not satisfied. Connecting the WDO pin to the KILL ‾ pin causes the ON pin to go low, disabling the HV buck and all LTC3375 regulators. The KILL ‾ pin can be pulled low by a pushbutton “paper clip” switch to power down all the regulators as a last resort.

AC–DC converters (rectifiers)

14.7 Three-phase controlled rectifier

Three-phase rectifiers provide a higher average output voltage compared to single-phase rectifiers. Figure 14.14 shows the schematic of three-phase controlled rectifier connected with highly inductive load. The thyristors are fired at an interval of 60˚. The frequency of the output ripple voltage is 6 f (f is the frequency of the AC supply). The filtering requirement is reduced as compared to a single-phase rectifier.

Figure 14.14 . Schematic: three-phase full-bridge controlled rectifier with R–L load.

Figure 14.15 shows the waveforms of the AC phase voltages and DC output voltage. Table 14.3 gives a summary of the thyristor switch conduction.

Figure 14.15 . Waveforms: three-phase full-bridge controlled rectifier with R–L load.

(a) AC phase voltages and (b) DC output voltage.

Table 14.3 . Summary of the thyristor switch conduction chart

Thyristors ON	T5–T6	T6–T1	T1–T2	T2–T3	T3–T4	T4–T5	T5–T6
Phase voltages	a, c, b	a, b, c	b, a, c	b, c, a	c, b, a	c, a, b	a, c, b
Duration (angle)	(0–30) + α	(30 + α) to (90 + α)	(90 + α) to (150 + α)	(150 + α) to (210 + α)	(210 + α) to (270 + α)	(270 + α) to (330 + α)	(330 + α) to 360

The equations of instantaneous phase and line voltages are,

The average output voltage is,

The RMS value of the output voltage is,

Fourier series of the AC supply side current connected to a highly inductive load via a three-phase full-bridge controlled rectifier is,

60V/3A step-down DC/DC converter maintains high efficiency over a wide input range

Introduction

Today’s high voltage applications—such as automotive, industrial and FireWire peripherals—place increasing demands on power supplies. They must provide high power, high efficiency and low noise, in a small space and over an ever-widening range of operational input voltages. Many high voltage DC/DC converter solutions can meet some of these conditions at high input voltages but they are unable to maintain high efficiencies at lower input voltages. Many of these same converters have frequency compensation schemes that require bulky input and output capacitors, which not only increase the size of the overall solution but also result in high output ripple voltage . The LT3430 is designed to alleviate all of these problems.

The LT3430 is a monolithic step-down DC/DC converter which utilizes a 3A peak switch current limit and has the ability to operate with a 60V input. The LT3430 runs at a fixed frequency of 200kHz and is housed in a small thermally enhanced 16-pin TSSOP package enabling it to save space while optimizing thermal management. Its 5.5V to 60V input range makes the LT3430 ideal for FireWire peripherals (typically 8V to 40V input), as well as automotive systems requiring 12V, 24V and 42V input voltages (with the ability to survive load dump transients as high as 60V). Furthermore, it was designed to maintain excellent efficiencies with both high and low input-to-output voltage differentials. Its current mode architecture adds flexible frequency compensation allowing the use of a ceramic output capacitor—resulting in small solutions with extremely low output ripple voltage (see Figures 70.3 and 70.4 ). Other features include a shutdown pin, which has an accurate 2.38V undervoltage lockout threshold and a 0.4V threshold for micropower shutdown (drawing only 25μA), and a SYNC pin, which allows the LT3430 to be synchronized up to 700kHz.

Theoritical considerations for buck mode switching regulators

Carl Nelson , in Analog Circuit Design , 2013

Output Ripple Voltage

Output ripple on a tapped-inductor converter is higher than a simple buck converter because a square wave of current is superimposed on the normal triangular current fed to the output. Peak-to-peak ripple current delivered to the output is:

A conservative approximation of RMS ripple current is one-half of peak-to-peak current.

Output ripple voltage is simply the ESR of the output capacitor multiplied times I _P-P. In this example, with ESR = 0.03Ω

This high value of ripple current and voltage requires some thought about the output capacitor. To avoid an excessively large capacitor, several smaller units are paralleled to achieve a combined 5.7A ripple current rating. The ripple voltage is still a problem for many applications. However, to reduce ripple voltage to 50mV would require an ESR of less than 0.005W—an impractical value. Instead, an output filter is added which attenuates ripple by more than 20:1.

LT1070 design manual

Carl Nelson , in Analog Circuit Design , 2011

Inductor

The energy storage inductor in a buck regulator functions as both an energy conversion element and as an output ripple filter. This double duty often saves the cost of an additional output filter, but it complicates the process of finding a good compromise for the value of the inductor. Large values give maximum power output and low output ripple voltage , but they also can be bulky and give poor transient response. A reasonable starting point is to select a maximum peak-to-peak ripple current, (ΔI). This yields a value for L1 of:

f = LT1070 operating frequency ≈ 40kHz

ΔI = peak-to-peak inductor ripple current

With the circuit shown, V_IN = 16V, V_OUT = 5V and ΔI set at 20% of 3.5A = 0.7A:

The ripple current in L1 reduces the maximum output current by one-half ΔI. For lower output currents this is no problem, but for maximum output power, L1 may be raised by a factor of two to three. For lower output powers, L1 can be reduced to save on size and cost. Discontinuous mode operation will occur even near full load if L1 is reduced far enough. The LT1070 is not affected by discontinuous operation per se, but maximum output power is significantly reduced in discontinuous mode designs:

I_P = LT1070 peak switch current

With L1 = 10μH, for instance, and I_P = 5A:

Efficiency is also reduced with discontinuous operation because of increased switch dissipation.

The load current where a buck regulator changes from continuous to discontinuous operation is:

With a 100μH value of L1, inductor current will go discontinuous at:

I_CRIT can never exceed 2.5A (one half maximum LT1070 switch current).

Peak inductor current in a buck regulator with continuous mode operation is:

With I_OUT = 3.5A and L1 = 100μH:

The core used for L1 must be able to handle 3.93A peak current without saturating.

Peak inductor currents in discontinuous mode are much higher than output current:

For L1 = 10μH, I_OUT = 1A:

The 10μH inductor, at 1A output current, must be sized to handle 4.14A peak current.

Power Management

Hank Zumbahlen , with the engineering staff of Analog Devices , in Linear Circuit Design Handbook , 2008

Regulated Output Switched Capacitor Voltage Converters

Adding regulation to the simple switched capacitor voltage converter greatly enhances its usefulness in many applications. There are three general techniques for adding regulation to a switched capacitor converter. The most straightforward is to follow the switched capacitor inverter/doubler with a LDO linear regulator. The LDO provides the regulated output and also reduces the ripple of the switched capacitor converter. This approach, however, adds complexity and reduces the available output voltage by the dropout voltage of the LDO.

Another approach to regulation is to vary the duty cycle of the switch control signal with the output of an error amplifier which compares the output voltage with a reference. This technique is similar to that used in inductor-based switching regulators and requires the addition of a PWM and appropriate control circuitry. However, this approach is highly nonlinear and requires long time constants (i.e., lossy components) in order to maintain good regulation control.

By far the simplest and most effective method for achieving regulation in a switched capacitor voltage converter is to use an error amplifier to control the on-resistance of one of the switches as shown in Figure 9-76 , a block diagram of the ADP3603/ADP3604/ADP3605 voltage inverters. These devices offer a regulated −3 V output for an input voltage of +4.5 to +6 V. The output is sensed and fed back into the device via the V_SENSE pin. Output regulation is accomplished by varying the on-resistance of one of the MOSFET switches as shown by control signal labeled “R_ON CONTROL” in the diagram. This signal accomplishes the switching of the MOSFET as well as controlling the on-resistance.

Figure 9-76: . ADP3603/3604/3605 regulated −3 V output voltage inverters

A typical application circuit for the ADP3603/ADP3604/ADP3605 series is shown in Figure 9-77 . In the normal mode of operation, the SHUTDOWN pin should be connected to ground. The 10 μF capacitors should have ESRs of less than 150 mΩ, and values of 4.7 μF can be used at the expense of slightly higher output ripple voltage . The equations for ripple voltage shown in Figure 9-72 also apply to the ADP3603/ADP3604/ADP3605. Using the values shown, typical ripple voltage ranges from 25 to 60 mV as the output current varies over its allowable range.

Figure 9-77: . ADP3603/3604/3605 application circuit for −3 V operation

The regulated output voltage of the ADP3603/ADP3604/ADP3605 series can varied between −3 V and −V_IN by connecting a resistor between the output and the V_SENSE pin as shown in the diagram. Regulation will be maintained for output currents up to about 30 mA. The value of the resistor is calculated from the following equation:

The devices can be made to operate as standard inverters providing an unregulated output voltage if the V_SENSE pin is simply connected to ground.

A typical application circuit is shown in Figure 9-78 . The Schottky diode connecting the input to the output is required for proper operation during start-up and shutdown. If V_SENSE is connected to ground, the devices operate as unregulated voltage doublers.

Figure 9-78: . ADP3607 application circuit

The output voltage of each device can be adjusted with an external resistor. The equation which relates output voltage to the resistor value for the ADP3607 is given by:

The ADP3607 should be operated with an output voltage of at least 3 V in order to maintain regulation.

Although the ADP3607-5 is optimized for an output voltage of 5 V, its output voltage can be adjusted between 5 V and 2 × V_IN with an external resistor using the equation:

When using either the ADP3607 or the ADP3607-5 in the adjustable mode, the output current should be no greater than 30 mA in order to maintain good regulation.

The circuit shown in Figure 9-79 generates a regulated 12 V output from a 5 V input using the ADP3607-5 in a voltage tripler application. Operation is as follows. First assume that the V_SENSE pin of the ADP3607-5 is grounded and that the resistor R is not connected. The output of the ADP3607-5 is an unregulated voltage equal to 2 × V_IN. The voltage at the Cp+ pin of the ADP3607-5 is a square wave with a minimum value of V_IN and a maximum value of 2 × V_IN. When the voltage at Cp+ is V_IN, capacitor C₂ is charged to V_IN (less the D1 diode drop) from V_OUT1 via diode D1. When the voltage at Cp+ is 2 × V_IN, the output capacitor C₄ is charged to a voltage 3 × V_IN (less the diode drops of D1 and D2). The final unregulated output voltage of the circuit, V_OUT2, is therefore approximately 3 × V_IN −2 × V_D, where V_D is the Schottky diode voltage drop.

Figure 9-79: . Regulated +12 V from a +5 V input

The addition of the feedback resistor, R, ensures that the output is regulated for values of V_OUT2 between 2 × V_IN −2 × V_D and 3 × V_IN −2 × V_D. Choosing R = 33.2 kΩ yields an output voltage V_OUT2 of + 12 V for a nominal input voltage of + 5 V. Regulation is maintained for output currents up to approximately 20 mA.

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