TOPswitch RCD clamp dissipation?

Posted by: treez on 06/29/2023

Page 19 of the TOP264 datasheet says…The RCD Clamp dissipation can be minimized by reducing the external current limit as a function of input line voltage (see Figure 19).

How can this be so?

As you know, For a DCM Flyback, the power input is 0.5*L*i^2*f.

And since the frequency would be the same , whether or not the high value resistor had been fed from rail to X pin, then the clamp dissipation would be the same , because the peak primary current would be the same, and the frequency the same?...so how coudl the RCD dissipation be less when the "external peak current limit thingy" had been done?

Comments

Hi.

Thank you for showing interest in Power Integrations' products.

Using the X-pin to set the current limit externally as shown in Figure 19 will reduce the peak primary current.

Please refer to Figure 36 and 37 for more information with choosing the RIL (external current limit resistor).

Along with the reduction in peak primary current, the primary clamp dissipation will also decrease.

Thanks, do you agree, if primary peak current decreases , then switching frequency must increase to keep the same power level....but how it increase?...TOPswitch is fixed to 132kHz

TOPSwitch implements a multi-mode PWM control. It is not a fixed frequency controller.

Duty and frequency will be modulated within the device's specifications to deliver the power required for a given peak primary current.

Please refer to Figure 6 for more information regarding multi-mode operation.

Thanks, but at maximum power, the switching frequency is 132khz...it cant go any higher than this.........it only goes lower than that when the power reduces down from max?

Ayk, Switching frequency doesnt start reducing till you go <66% of max power.

Depending on the resistor on the X-pin, the switching frequency at full frequency PWM mode will vary.

For example, if you connect the X-pin to the CONTROL pin, the maximum frequency will be at 66kHz.

You will have to choose a maximum frequency of operation and decide on the appropriate peak primary current required to deliver the output power.

The RCD dissipation decreasing due to lower peak primary current is demonstrated by the equation you provided.

Thanks, yes, i appreciate that, but at the end of the day, you have to have the peak current high enough to feed the load at its maximum power.

As discussed, f(sw) is fixed when at max power, to either 132 or 66khz.....so we cannot reduce the peak current when we are at max load....because we will see vout dropping out.

For a given output power specification, the number of design combinations that will be able to deliver the required output power can be more than one.

The statement from the datasheet only suggests that reducing the primary peak current will reduce the dissipation on the RCD clamp.

It is also possible to design a TOPSwitch-based power supply operating before the maximum frequency even at full load.

This may mean designing the power supply to deliver the maximum output power required with minimum frequency and peak primary current.

Please note that the TOPSwitch does not need to operate at full frequency mode to be able to deliver the required output power.