How to reduce current spike at inductor during start up ?

[Patrick66]

Hello everyone, how do we normally reduce the current spike of the flyback converter during start-up as shown in the figure below? Where V(n006) and I(R6) are the output voltage and current. Thank you for the help in advance.

[T3sl4co1l]

Simple: don't use voltage-mode control!

Look into current-mode control, particularly peak and average modes.

Tim

[PwrElectronics]

You are asking about the high currents in the coupled inductors/transformer?

Most power supply controllers will have some form of a softstart feature.  This limits the duty cycle until the output voltage reaches its target.  The newer types may have more elaborate control schemes for startup and response to faults and overloads but even the older ones from 30yrs ago usually had a softstart feature.

[Harry_22]

Install current sensor resistor between M1 Source and GND and limit the current.

[PwrElectronics]

Install current sensor resistor between M1 Source and GND and limit the current.

Yes, this is the modern approach.  Inner control loop on current with outer control loop on the voltage.

For the OP, they can add a peak current detector that will end the ON time of the primary switch when the peak current is reached.  Once past the inrush period, current will be less and then the outer voltage control takes over.

This also helps to prevent damaging the circuit when there is a overload or shorted load fault.

[PCB.Wiz]

Hello everyone, how do we normally reduce the current spike of the flyback converter during start-up as shown in the figure below?

That does not look too bad ?
The effect is  amplified because you have a 21V load offset, so your slight voltage overshoot give more current overshoot.

You look to be LED driving with current sense, if you think that current effect is really important, you can feed forward a little of the voltage rise, during ramp up.
(series RC from voltage to current opamp will manage that)

[David Hess]

If you do not use current mode control, then implement soft start by ramping the reference voltage applied to the error amplifier from zero.

[Patrick66]

This is current mode control LED driver.

[Harry_22]

Could you show the drain current rise diagram for one period (10µs) and also via L1, L2.
Take a point between 16-17ms from start.

[temperance]

This is current mode control LED driver.

Not really. Here is a reference describing current mode control.

https://www.ti.com/lit/an/snva555/snva555.pdf?ts=1716612463776&ref_url=https%253A%252F%252Fwww.google.com%252F