How to do load transient response test on power supply

The power supply is the core part of electronic products. As a hardware engineer, when the power supply is designed, load transient response testing of the power supply is usually an essential test step. Common electronic loads have transient response test functions.

When you don't have an electronic load on hand, you can make a simple test tool to measure the transient response function of the power supply. Here's how to test it.

The core of the load transient test tool is the MOSFET power switch controlled by the microcontroller. It works in on and off states with a certain duty cycle. When it is connected to the output end of the power supply, the load controlled by the MOSFET switch The resistor is inserted into the circuit intermittently, thus forming rapidly changing load pulses. The load step generated by this method changes very quickly (about 500ns rise/fall time) and can be used in testing power supplies with any output voltage. When such a step load is applied to the output of the power supply, we can analyze the stability of the control loop by measuring the waveform of the output voltage.

Rapidly changing load steps can impact the regulator's control loop over a wide frequency range. If the control loop is unstable or underdamped, ringing will appear on its output voltage waveform. Signal. This method is only effective in continuous conduction mode (CCM), so discontinuous conduction mode (PSM) needs to be avoided during the test, even the PSM-CCM conversion process must be avoided, so the static The load is adjusted to make the system work in CCM mode.

The following figures show the load step response waveforms of a poor and a good 3.3V/3A converter. In the waveform in the first picture, the output voltage of the power supply shows serious oscillation after the load jumps, which means that its control loop has obvious stability problems. In most cases, this is caused by a mismatch between the converter's loop compensation settings and the capacitance of the output capacitor.

In addition to loop stability issues, the path inductance in the PCB layout, the oscillation process at the power input terminal, etc. can also cause similar ringing phenomena. If we use fast load transient testing tools to test, these problems can be easily located.

The above picture shows a MOSFET driven by a microcontroller. The gate drive circuit of the MOSFET is set according to the switching speed of approximately 500ns rise/fall time. Reducing or canceling C2 can increase the switching speed. The actual load current change speed is also related to the size of the connection inductance between the test tool and the test object. When the test voltage is very low (such as <2V), it is best to use short and thick wires to connect the tool and the test object. .

JP1~JP7 are jumpers, used to select the required pulse load resistance.

The rise and fall times of current pulses can be measured with an oscilloscope. The circuit will produce parasitic self-inductance when it is working. Parasitic self-inductance is the main factor that limits the rise time of the current pulse. The formula for the self-inductance state is: V/L=di/dt. Here V represents the applied voltage and L represents the inductance value. , di/dt represents the rate of change of current per second. If the load current change rate of the power supply is required to be higher, the total self-inductance of the load must be smaller. Assuming that the power supply voltage is 3.3V, a di/dt of 10A/ns requires that the total self-inductance of the load does not exceed 0.33mH, so Low self-inductance requires precise selection of load resistors and reasonable circuit layout. The load resistor must be metal oxide, carbon film or contain other carbon components. Multiple load resistors can be connected in parallel. After multiple resistors are connected in parallel, the self-inductance is averaged to each resistor, thereby minimizing the additional self-inductance. It also reduces the rise time of the current. Any additional self-inductance will cause ringing. , and makes it difficult to measure waveforms with an oscilloscope.

We can adjust the frequency and duty cycle of the output PWM through the microcontroller, and test the response ability of the power supply by adjusting these parameters. Of course, we can also use NE555 instead of the microcontroller to achieve the same function.