Generating 12V PWM from 5V PWM

Question

I am generating a 5V PWM signal from my MCU (an Atmel ATMEGA1284P). I need to convert this to a 12V PWM signal. For this I designed the circuit below, something very similar to a Sziklai pair, but adapted to match this case. It uses a bipolar NPN transistor to drive a bipolar PNP transistor. Ideally, I would need V_LOAD to be a 12.0 V PWM.

I ran a few simulations and this seemed to fulfill the purpose really well, until I remembered C_LOAD. Since V_LOAD is not actively pulled down when CLK2 goes low, the fall time is very long (too long, about 25 μs).

Any ideas on how to improve my design? Or should I trash this and use something else?

I apologize in advance if this has been asked before.

Some clarifications: The maximal current I'm going to drive will be around 10 mA. I have some pretty harsh specifications to fulfill: V_LOAD should be 12.0V and the rise and fall times (10%-90% = 1.2V-10.8V) should maximally be 2 μs,

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EDIT: Thanks for your answers. Here's what I ended up using: Simulation.

It's based on Majenko's answer with two transistors added as gate drivers to reduce the time it takes for the PWM to go LO. Furthermore, MOSFET's with low gate capacitance have been chosen (BSS84P and BSS138). The fall time then becomes 3 μs.

Answer 1

What @IgnacioVacquez-Abrams is talking about in his comment is creating a Push-Pull output. This is basically what is in the output stage of your Microcontroller.

^{simulate this circuit – Schematic created using CircuitLab}

When the PWM is HIGH Q1 conducts. The gates of M1 and M2 are then both LOW, so M1 is on and M2 is off. RL and CL are effectively connected to +12V. When the PWM goes low Q1 stops conducting, the gates of M1 and M2 are pulled up to +12V by R1. M1 turns off, M2 turns on. RL and CL are then effectively connected to ground.

It is important to select MOSFETs M1 & M2 with gate threshold voltages sufficiently high so that both FETs are not ON when the gate voltage is in-between 12V & 0V. This is one place where you do NOT want to use logic-level MOSFETs.

Answer 2

How much current do you need?

There are two easy ways to do this with simple components. One method is good for only a few mA current - enough to feed the input of something active like a VFD. The other method is good for as much current as you want to make it for.

I'll show the higher-current option first.

^{simulate this circuit – Schematic created using CircuitLab}

The above circuit works very well and is good for medium-speed PWM (DC to tens of KHz). It can be beefed up substantially if needed but more components will be required.

Note that this first circuit inverts the logic signal: a HI on the input gives you a LO on the output.

This next circuit is just a level-shifter. It has the advantage of very low cost but it's only good for a few mA.

^{simulate this circuit}

This circuit functions like a common-base amplifier. The base is held at +5V when the input is HI (also +5V) and the transistor is OFF. This allows the collector to float up to the +12V rail.

When the input goes LO, the transistor turns on hard and pulls the collector to just above the logic LO level of the input. Note that all of the sinking output current goes through the input pin.

This circuit is non-inverting: a logic HI on the input gives you a logic HI on the output. This also works well - it's what I use to interface some of my stuff to VFDs that require an input signal higher than 5V.