[ §8 · calculators · switch thermals ]

Switch thermals

Conduction loss at the hot R_DS(on). Working from the cold figure understates it several times over: for the default device the two differ by 2.6×. The check is the one davekni uses: what thermal resistance you are allowed to spend, rather than guessing at a heatsink.

primary current, peak · Adevices per positionpositions in bridgeR_DS(on) cold · ΩR_DS(on) hot · Ωswitching loss · µJ eachfrequency · kHzburst length · µsduty · %allowed rise ΔT · K

whole bridge340 Waveraged over 14 % duty, 8 devices

per device, in burst304 W40.0 A RMS at the hot R of 0.19 Ω

transient Z_th0.329 K/Wover 2000 µs for ΔT 100 K

current per device 40.0 A RMS conduction, cold 115 W (R 0.072 Ω)conduction, HOT 304 W (R 0.19 Ω) ← use this they differ by ×2.6switching 0 W in burst 304 W per deviceaveraged (14 %) 43 W per deviceWHOLE BRIDGE (8) 340 W average, this is the heatsink To stay inside ΔT = 100 K: transient Z_th over 2000 µs at most 0.329 K/W static R_th (average) at most 2.35 K/W Requirements are moderate.Still check the transient impedance curve at your pulse width.

formula

I_RMS per position = I_pk/2 (it conducts half a sine)   per device = /n
P_cond = I_RMS² · R        P_sw = E_off · f
allowed Z_th(pulse) = ΔT / P_burst    allowed R_th(steady) = ΔT / P_average

limits

Switching loss is entered by hand, because with ZVS working it is close to nothing and when ZVS breaks down it goes through the roof, and there is no predicting it. Leave it at 0 to see the conduction loss on its own, and read that as the floor: if ZVS fails, this margin is gone. It is another reason to set the phase lead generously.

The allowed Z_th must be checked against the transient thermal impedance curve in the datasheet at your pulse width, not against the static R_th(j-c).

davekni's rule of thumb for the order of magnitude: devices fail at about 4× the continuous or 2× the pulsed rated current.