# Problem in a boost calculator circuit

In order to get an output of 100V-300v (for the ultrasound piezo drive ) from a 3.3V output of the microcontroller. I have built the boost circuit according to this circuit: But there is one point that I donnot quite understand- the +UB. I thought that it's the output, so I connect it to the oscilloscope, and I got the results as showed in the picture. If +UB is the output, I can only have 10V as maximum as provided by Vcc. Can you help me to find the problem in my circuit ? Thanks!

The Maximum Repetitive Reverse Voltage of the 1N5817 is 20V and no-doubt your simulator will take this into account. This means that you cannot create an output any greater than 20V. Think about it - to generate 300V, that diode has to be able to withstand a reverse breakdown voltage of at least 300 volts.

Another problem is that in 0.5 msecs the inductors current will have reached 2.27 amps and you are expecting a 1A diode to pass that energy thru to the output capacitors: -

V = L di/dt therefore di = V dt/L = $\dfrac{10\times 0.0005}{0.0022}$ = 2.27 amps.

the 2.27 amps is the current attained thru the inductor when the MOSFET is switched on for 0.5 msecs - this current is then forced to flow thru the 1N5817 when the transistor open circuits. Clearly you are exceeding the limits on the diode twice.

Next, the IRFD110 is only rated to have 100V max on drain wrt source so you can never get 300V without damage. Also, with an on resistance of 0.54 ohms and an avarage current of maybe 1A when conducting, the losses are going to be noteworthy if not excessive.

• Thanks for your answer andy! actually, when I changed the diode, the output really grows to more than 30v, But the whole process is too slow...take about 5 minutes...do you know why ? – user5313224 Sep 10 '15 at 16:18
• Another problem is that we only have an output of -1V~1V, so it seems donn't work so well as a boost input. Finally, in order to drive the piezo, we need an -30v~ 0v negative square wave, while this boost calculator only give an output of high voltage DC. Do you know how to solve this problem, to convert the 30v DC to -30v square wave with a certain bandwidth? Thanks a lot! – user5313224 Sep 10 '15 at 17:08
• Unless you are not showing something in the circuit like a load resistor I can't explain point 1. You should be transferring about 5mJ every cycle and the 33uF needs about 165 mJ to be at 100 volts so maybe you are not telling me about a load resistor? If you want ac to drive a piezo directly I would design a ferrite transformer and low voltage power oscillator to drive the step up transformer, that in-turn drives the piezo. I think a transformer is the right way. – Andy aka Sep 10 '15 at 17:22