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Half bridge: Totally different results with ideal switch vs MOSFETs

Javier1234

Hello all,

I'm new to PSPICE and I'm trying to simulate a simple half-bridge inverter. When I simulate the topology with ideal switches, the output voltage corresponds to a train of rectangular pulses with a height of Vdc (like it should). But when I simulatethe same topology using MOSFETs (two IRF150 MOSFETs) the output is a train of rectangular pulses of roughly 12% of Vdc. Can it be that the MOSFETs have such a high voltage drop in PSPICE, or am I modelling the circuit wrong??

Here is the netlist from the ideal circuit:

 D_D3         $N_0001 $N_0002 D1N4002
S_S2         $N_0001 0 0 0 Sbreak
RS_S2        0 0 1G
D_D4         0 $N_0001 D1N4002
V_V4         $N_0002 0 100
V_V3         $N_0003 0  
+PULSE 0 15 0.55ms 0.01ms 0.01ms 0.45ms 0.9ms
V_V1         $N_0004 0  
+PULSE 0 15 0.1ms 0.01ms 0.01ms 0.45ms 0.9ms
S_S1         $N_0002 $N_0001 $N_0005 0 Sbreak
RS_S1        $N_0005 0 1G
R_R_Load         0 $N_0001  1k  
R_RG1         $N_0004 $N_0005  10  
R_RG2         $N_0003 0  10  

and here the one with the MOSFETs

 D_D3         $N_0001 $N_0002 D1N4002
D_D4         0 $N_0001 D1N4002
V_V4         $N_0002 0 100
V_V3         $N_0003 0  
+PULSE 0 15 0.55ms 0.01ms 0.01ms 0.45ms 0.9ms
V_V1         $N_0004 0  
+PULSE 0 15 0.1ms 0.01ms 0.01ms 0.45ms 0.9ms
R_R_Load         0 $N_0001  1k  
R_RG1         $N_0004 $N_0005  10  
M_M3         $N_0002 $N_0005 $N_0001 $N_0001 IRF150
R_RG2         $N_0003 $N_0006  10  
M_M4         $N_0001 $N_0006 0 0 IRF150

The only difference between the two are the MOSFETS M_M3 and M_M4 instead of the switches S_S1 and S_S2. This thing is driving me totally nuts, any help would be greatly appreciated.

Cheers!

Javier

  • Javier1234
    And here is the comparison of the voltage waveforms, the one at the top is the circuit with ideal switches and the one in the bottom the one with MOSFETs
  • oldmouldy

    I think that your problem is that the M3 gate is "clamped to" 0, you probably need some offset for that Vpulse, or connect that Vpulse between the Gate and Source of M3. The "switch" isolates the switching voltage and you have the switches common to 0 so the switch "sees" the voltage to operate the switch, the MOSFET needs Vgs to switch, that's OK for M4 but not for M3.

  • Javier1234
    thanks a lot oldmouldy, that was exactly my error.
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