Transient response testing tools

This design replicates a Liqi load transient testing tool: Function
 
 
Introduction:
This design generates very fast load steps by rapidly switching resistors on and off using an MCU-driven MOS transistor. It can be used to quickly verify the pulse load behavior of a voltage regulator and to check the stability, layout resonance, and input power ringing of a DC/DC converter. Various pulse load resistors can be selected via jumpers, and the static load level can be set using an adjustable power resistor. The pulse load frequency and duty cycle can be adjusted via buttons. By measuring the regulator output voltage during a fast load step, the behavior of the regulator control loop can be observed.
 
In simpler terms: The MCU generates a PWM signal with an adjustable duty cycle and frequency, which drives an N-MOS transistor via an inverter as the gate driver to generate a pulse load. The load size can be switched by controlling the resistor connected to the circuit using jumpers.
A single 18650 battery is used for power supply, isolated from the power supply under test for ease of use. An onboard LED indicator displays the battery level; it needs to be removed for charging when the battery is low.
 
Instructions for Use:
The self-locking switch PWR controls the power supply of the entire circuit.
The D+ and D- buttons control the duty cycle, increasing or decreasing by 5% with each press, up to a maximum of 50%. Note that the power on each resistor should not exceed 1W.
The Freq button controls the PWM frequency, switching between four preset frequencies.
The red LED indicates the battery status: off at 4.2-4V, flashing slowly at 3.8-4V, flashing quickly at 3.6-3.8V, and constantly lit below 3.6V, requiring charging.
The blue LED is a constant power indicator.
 
For detailed power supply testing instructions, please refer to Liqi's explanation in the link above; it is very detailed and will not be repeated here.
The attachment contains the compiled program and source project.