Overshoot & Undershoot in DC-DC Converters — Why They Happen and How to Fix Them. If you’ve ever debugged a DC-DC converter, you know the story: The output looks perfect at steady state, but under fast load or input changes… suddenly you see voltage overshoot, undershoot, or current spikes on the scope. What Are Overshoot & Undershoot? *Voltage Overshoot → Output voltage rises above its setpoint (risking component stress). *Voltage Undershoot → Output voltage dips below its setpoint (risking system resets or malfunctions). *Current Overshoot → Inductor or load current spikes above expected levels. *Current Undershoot → Current briefly falls too low, affecting energy delivery to the load. These transients are short-lived, but their impact on reliability, EMI, and performance can be significant. How to Minimize Overshoot & Undershoot: Proper Control Loop Compensation: *Ensure enough phase margin and bandwidth. An underdamped loop = ringing and overshoot. Use Soft-Start & Pre-Bias Techniques: *Ramp the duty cycle gradually to avoid inrush spikes. Choose Adequate Output Capacitors: *Low-ESR capacitors help absorb transient energy quickly. Add Snubbers or RC Dampers: *Reduce voltage ringing due to parasitic inductances. Optimize PCB Layout: *Short, wide traces in the power loop minimize parasitic inductance → less overshoot/undershoot. Employ Feed-Forward / Active Control: *Advanced controllers can anticipate load or line changes instead of merely reacting. Consider Current Slew Rate Control: *Especially in high-power converters, limiting di/dt smooths transient responses. Takeaway: Overshoot and undershoot directly impact converter reliability and end-user experience. Through smart control design, component selection, and PCB layout, you can build converters that stay stable even under the toughest transients.
Overshoots and undershoots are bound to happen due to a sudden load transient (for eg 50-100%) and finite output impedance of the converter. It is a product of the magnitude of load transient and the worst case output impedance (roughly = 1/(2pifC) where f is the cross over frequency). As such, they are not a problem as long as they meet the converter/load transient requirements. What matters is the recovery from such a transient and this indicates the relative stability of the loop and gives a qualitative idea about its phase margin. Having electrolytic caps at the output help to limit these over/undershoots in case of low BW converters. Their high ESR can help stabilize the loop by providing a zero. However, care must be taken in shaping the loop as this zero may reduce the gain margen and hence, ripple rejection.
RHPZ
“Nice explanation. Also, selecting switches appropriately with respect to Miller effect and Safe Operating Area (SOA) will help minimize overshoot issues.”
Manager - Power Electronics Design ( EV R&D) at TATA AutoComp | Developing EV Chargers and DCDC converters | Motor Inverter Development | Solar Optimizer | Fuel Cell
2moOvershoots and undershoots are commonly observed during load transient tests of DC-DC converters. A standard test involves applying a 30%-80%-30% load step to evaluate the converter’s transient response. This test helps determine key performance metrics such as response time, voltage undershoot during load increase, and voltage overshoot during load decrease.