# How to calullate RMS current through switch (MOSFET) in boost converter?

Trying to calculate values for a boost converter based on document Basic Calculation of a Boost Converter's Power Stage.

Want to know power dissipation though MOSFET which that document does not mention.

It mentions maximum switching current:

$$I_{SW(max)} = \frac{\Delta I_L}{2} + \frac{I_{OUT(max)}}{1-D}$$

where:

• $$\\Delta I_L\$$ - ripple current trough inductor
• $$\I_{OUT(max)}\$$ - maximum output current
• $$\D\$$ - duty cycle

How to calculate RMS switching current through MOSFET so power dissipation can be calculated?

• Could it be simply Iout? – Marko Buršič Aug 31 '19 at 6:49

The RMS current is the equivalent DC current, so if you switch 1 Amp of current with a 50% duty cycle, your RMS current would be 707mA,

However this is not the only part that causes power dissipation in mosfets, there is also switching losses, to do with how when the mosfet is switching on or off, there is a breif moment each time when its not fully on or off, but rather acting as a resistor,

Boost converters depending on there topology may also see some heating through the use of its body diode instead of an external diode, but this is rarer these days.

• Theory 101 needs to be retaken here. 1 amp switched at 50% duty is an RMS 0.7071 amps. – Andy aka Aug 31 '19 at 8:47

If you want to estimate the conduction losses, you can find the RMS current during each segment of the waveform (in continuous or discontinuous mode), square each, find the mean and take the square root.

The theoretical waveforms are triangles maybe plus a bias, which can be solved for RMS in closed form.

In reality, unless this was a class problem set, you'd probably just do it numerically, for example with LTspice (free) or MATLAB (not free) or one of the latter's open source alternatives.