Prevent DC-DC Converter Oscillations With Input Capacitance

These big caps aid power stability as seen at the input to a switching regulator.

Switching regulator circuits (buck, boost, buck-boost, flyback, etc.) in their most basic forms are quite simple. They use an inductive element, isolation (in the case of flyback converters), output capacitors for power stability, and a rectifying element like a diode. If you look at the typical switching regulator waveforms, it is implied that everything should exhibit the desired stable DC output behavior with low switching noise and operation in continuous conduction mode.

What’s wrong with this picture, and where are the basic tutorials on switching regulators incomplete? The answer is in the behavior of transients in these circuits. Transient responses can be addressed in three areas of the circuit:

• At the output, where inductor and capacitor selection are used to reduce switching noise
• At the switching node, where the MOSFET pin inductance and gate capacitance lead to ringing
• At the input, where the switching stage draws fast pulses of current from the input power source

In this article, we’ll look at the 3rd point above, where noise occurs in the input power net due to switching action in the converter. As it turns out, the behavior seen in a DC-DC converter module will be very similar to that seen in a PDN for a high-speed PCB, where oscillations can occur on the input power rail. That hints at the solution to ensure power stability: more capacitance!

Why Place Input Capacitance on a Power Regulator?

As was stated in the introduction, the need for capacitance on the input ports of a DC-DC converter arises due to the need to ensure power stability. Capacitors store charge that has some electrical potential energy, and that energy can be delivered to a load when the capacitor terminals and the load are connected. The power source providing the input voltage for your voltage regulator circuit must be able to respond quickly to changing power demands as the converter operates for the input power bus to be stable.

Power supplies, power converter modules, and batteries might not have low enough impedance and time constant to respond to current demands in the switching stage. Even if there is a terminating capacitance on the supply output, the output connections and wire leads can add resistance to the supply’s output impedance. However, there is a simple solution: add capacitance!

Placing input caps on the PCB when a power regulator has a slowly-responding (possibly unregulated) input power source provides a much faster response. In effect, the switching regulator draws power from the input capacitors, and the input power module simply recharges the input capacitance.

Power delivery to an input capacitor and switching stage.

Effects of Adding Input Capacitance

We can see the importance of adding enough input capacitance when we look at the input bus voltage during switching. Consider the buck converter design shown below. In this regulator, we’ve assumed that the input 12 V power supply has some capacitance on the output (10 uF), but the connectors, wire leads, terminals on the PCB, and imperfections in manufacturing lead to an additional 500 mOhms of resistance on the output terminals.

Example buck converter design with output impedance modeled with a resistor and capacitor.

If we look at the SPICE simulation results for this circuit, we see the following graphs. The transient response is very large as viewed at the inductor current (in green), the input voltage (in blue), and the output voltage (in red). Large overshoot and input power variations of 500 mV can be seen in these graphs.

SPICE results for the above buck converter design example.

Now let’s see what happens if we add some capacitance (22 uF) across the input terminals of the high-side MOSFET and GND. This is not a huge input capacitance, but it does provide a lot of benefit for power stability, as we will see below. On the PCB, this capacitor should be placed very close to the MOSFET terminals.

Modified switching regulator with an input capacitor.

The transient analysis results below nicely show that the transient response on the input is stabilized. This is provided by the filtering action of the input capacitor C3. There is still some turn-on time allowance in this circuit, but the turn-on time is still quite fast at 0.5-1 ms.

Transient analysis results with the modified switching regulator.

There is some effect on the switching current in the inductor and the output voltage, but we can accept a very slightly increased inductor transient response in exchange for extremely high power stability. Other design decisions you could make in this circuit to increase the stability of the circuit would be to:

• Apply a snubber across the low-side MOSFET terminals to dampen any ringing on the PWM waveform and switching node voltage
• Apply a very small series resistance to the output capacitor C1 to slightly dampen its transient response
• Apply another output filter stage that is carefully designed to prevent creation of a new pole in the regulator’s transfer function

To determine exactly how much capacitance you need, you can use the same ideas applied in sizing decoupling capacitors: determine the amount of charge or current that needs to be supplied within 1 time constant, and calculate the capacitance given the applied voltage (12 V in the example above).

Summary of Power Stability Strategies

When you start digging deep, any switching regulator circuit will be a relatively complex circuit that requires plenty of analysis to properly design. Problems with output transients and switching noise are interrelated, so a plurality of strategies is needed to produce a circuit and PCB layout that gives the required stability and noise reduction. The table below shows a set of strategies used to combat various noise sources in a DC-DC converter.

DC-DC converters can get much more complex as current requirements increase, and the increased complexity brings more opportunities to produce noise and transients at various points in the circuit. Thorough circuit simulations are needed to identify the sensitive areas of these circuits. Once you’ve identified noise-prone portions of the switching regulator, make sure you design the PCB stackup and routing to ensure parasitics are controlled so that new noise is not introduced into the design.

When you’re ready to simulate your DC-DC converter circuit and you need to determine input capacitance requirements, the industry’s best circuit design and simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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