# Help! circuit simulation

#### declan2693

Joined Feb 2, 2018
37
Hi,

Really need help. Could someone please simulate this circuit for me(on any simulator). I've tried 100's of times on PSIM but it doesnt work for some reason.
Here is the schematic. Ill upload my circuit also to show ive worked on it and not lazy. There is a problem with my switches. I've entered the correct duty cycle and such. The switch ive cycled wont turn on no matter what input voltage I put in it.

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#### Alec_t

Joined Sep 17, 2013
12,198
I've entered the correct duty cycle and such.
All I see is the number '1000000'. How does that indicate duty cycle, delay, on time, off time, cycle period etc?

#### declan2693

Joined Feb 2, 2018
37
All I see is the number '1000000'. How does that indicate duty cycle, delay, on time, off time, cycle period etc?
Thats is the frequency. The duty cycle for V2 mosfet is 0.54. The duty cycle of the circled switch and the outputs is 0.46 with a phase delay of 194.4

#### crutschow

Joined Mar 14, 2008
27,708
Need to know the frequency, duty-cycle, and voltage swing of all the voltage sources controlling the MOSFETs.
And if there's a delay between signals give it in time, not degrees.

#### declan2693

Joined Feb 2, 2018
37
Need to know the frequency, duty-cycle, and voltage swing of all the voltage sources controlling the MOSFETs.
And if there's a delay between signals give it in time, not degrees.
frequency is 1Mhz, duty cycle for V1,V6 switches is 0.54 so the other three are 0.46. The voltage input of all control sources is 1.5V. The mosfet threshold is 1V. Delay is 0.54/1Mhz

#### crutschow

Joined Mar 14, 2008
27,708
duty cycle for V1,V6 switches is 0.54
Does that mean they are both on at the same time?

#### declan2693

Joined Feb 2, 2018
37

#### Marc Sugrue

Joined Jan 19, 2018
217
You may need some series resistance from the PWM source to the NMOS Gate.

#### Marc Sugrue

Joined Jan 19, 2018
217
Another thing to note the NMOS FET shown in your schematic shows a reverse diode accross the Drain-Source which could be giving you a reverse path for your output caps to discharge (shown as negative volts on your waveforms i believe). The book circuit you attach treat the switch as hard on or hard off switches with no reverse potential.

#### crutschow

Joined Mar 14, 2008
27,708
Then that means the input is being shorted to ground through the V1 switch and the V6 switch.
(Why aren't the MOSFETs labeled?)

#### ebp

Joined Feb 8, 2018
2,332
As Marc points out, the two right hand switches cannot be conventional MOSFETs, either N or P channel. These switches must be implemented using pairs of FETs - source to source or drain to drain

The circuit must operate in discontinuous inductor current mode or the boost and buck functions interact, getting current intended for the other side. Timing must be correct to avoid forced continuous mode at light load and to avoid disconnecting both ends of the inductor if it is not fully discharged.

The left FET must be ON continuously throughout the boost cycle, from the time the boost switch is turned on until the inductor is discharged.

There is a reason this circuit is not used. It is a very complicated way to do a poor job of something done much better otherwise.

#### declan2693

Joined Feb 2, 2018
37
As Marc points out, the two right hand switches cannot be conventional MOSFETs, either N or P channel. These switches must be implemented using pairs of FETs - source to source or drain to drain

The circuit must operate in discontinuous inductor current mode or the boost and buck functions interact, getting current intended for the other side. Timing must be correct to avoid forced continuous mode at light load and to avoid disconnecting both ends of the inductor if it is not fully discharged.

The left FET must be ON continuously throughout the boost cycle, from the time the boost switch is turned on until the inductor is discharged.

There is a reason this circuit is not used. It is a very complicated way to do a poor job of something done much better otherwise.
Ok thanks for that. You say this circuit is not used. Do you have a better schematic for a single input dual output buck boost converter with four switches? I am trying to simulate something close to the TPS65135. I will keep at this.

Thanks to Marc also.

#### declan2693

Joined Feb 2, 2018
37
Hi guys,

I was talking to someone and they said I need to change from n-mosfet to p-mosfet for V2 and V6. Would that be correct?

#### Marc Sugrue

Joined Jan 19, 2018
217
Hi guys,

I was talking to someone and they said I need to change from n-mosfet to p-mosfet for V2 and V6. Would that be correct?
You can use either as long as you satisfy the drive requirement for the FET

#### ebp

Joined Feb 8, 2018
2,332
OK, I'm now confused as to the objective of the circuit.
Because of the name previously used, I took the intent to be to produce one output that is greater in voltage magnitude than the input and one that is less, both with the same sign as the input e.g. +10 V in, +5 V and +20 V out. The TPS65135 is intended to produce positive and negative outputs, which is something quite different. The inductor "is discharged to both of the outputs simultaneously...", so my previous remarks do no apply.

#### Marc Sugrue

Joined Jan 19, 2018
217
Hi Declan, page 8 of the TPS65135 gives you a block circuit which uses NMOS so you are on the right path.

#### declan2693

Joined Feb 2, 2018
37
OK, I'm now confused as to the objective of the circuit.
Because of the name previously used, I took the intent to be to produce one output that is greater in voltage magnitude than the input and one that is less, both with the same sign as the input e.g. +10 V in, +5 V and +20 V out. The TPS65135 is intended to produce positive and negative outputs, which is something quite different. The inductor "is discharged to both of the outputs simultaneously...", so my previous remarks do no apply.
Hi, so basically I dont know how to do the schematic of the TPS65135 so I found the circuit show and trying to get that schematic I showed working before trying that