Driver circuit not controllable, heating up

1. What Olin says technically (as almost always :-)). Also, as he says - you need to be clearer in describing your need if you want a good answer.

2. Lots of circuits here plus various pictures (this is just a Google image search - but useful). Be aware that the quality of the offerings will range from very good through rubbish. Caveat Emptor.

   Here are a few samples from above. No guarantees !!!

   • Radio Shack LASER pointer driver

   • Roithner EU38 LASER Diode driver

   • Basic driver that seems to do what you want

   • EDN article

3. This is a superb LASER resource - hav a good look at what it offers. Sam's LASER FAQ is a long time well known site with much good information. Here is his Diode LASER power supplies page.

   From the above, see at the end of this post "Care and feeding of LASER diodes".

4. Commercial LASER Diode Driver PCBs. These LOOK like they could be modestly priced if they chose to make them so - but they are probably horriobly over priced.

5. This is for information only - worth a look. It shows what goies into a commercial LASER diode driver block diagram - scary stuff.

From Sam's LASER Diode page - reformated:

Care and feeding of LASER diodes.

The following must be achieved to properly drive a laser diode and not ruin it in short order:

• Absolute current limiting. This includes immunity to power line transients as well as those that may occur during power-on and power-off cycling. The parameters of many electronic components like ICs are rarely specified during periods of changing input power. Special laser diode drive chips are available which meet these requirements but a common op-amp may not be suitable without extreme care in circuit design - if at all.

• Current regulation. Efficiency and optical power output of a laser diode goes up with decreasing temperature. This means that without optical feedback, a laser diode switched on and adjusted at room temperature will have reduced output once it warms up. Conversely, if the current is set up after the laser diode has warmed up, it will likely blow out the next time it is switched on at room temperature if there is no optical feedback based regulation.

• Note that the damage from improper drive is not only due to thermal effects (though overheating is also possible) but due to exceeding the maximum optical power density (E/M field gradients?) at one of the end facets (mirrors) - and thus the nearly iSnstantaneous nature of the risk.

• The optical output of a laser diode also declines as it heats up. This is reversible as long as no actual thermal damage has taken place. However, facet damage due to exceeding the optical output specifications is permanent. The result may be an expensive LED or (possibly greatly) reduced laser emission.

Your driver IC will work with insulated gate devices (MOSFETs, IGBTs). Your Schematic shows BJTs.

BJTs require a current all the time they are on, MOSFETs or IGBTs need just a charging current into their gates and will stay on once the gate is charged. Using BJTs, your bootstrap capacitors will be discharged quite quickly.

Now...

a) The bootstrap cap needs to be able to hold its voltage without much loss when the gate of the power transistor is charged, i.e. it needs to be way bigger than the gate capacitance. 100 nF sounds good, because typical gate capacitance values are between 500 pF and 2 nF. Also, it needs to be able to hold up the supply current of the driver while it tries to keep its high output signal alive. The current will be small unless you use a BJT (see my intro above). However, 1 Hz is very slow. Assuming your CLK1 signal is symmetrical, the bootstrap caps need to supply the high side drivers for half a second. Typical PWM signals for such bootstrapped applications run at or above 10 kHz, i.e. the high or low times are 10000 times faster than in your simulation.

b) The bootstrap diode needs to be able to block the supply voltage (100 V in your case). Also, it should be of a fast switching type because it needs to block quickly once the lower power transistor turns off. Slow diodes will affect the switching behavior of the power transistors - i.e: add considerable switching losses. A 150 V / 1 A Schottky diode or a 200 V / 1 A ultrafast silicon diode sound like reasonable choices.

c) In the ideal world, your drivers' supply (12 V) will always feed the driver ICs, no matter what. A real circuit will have some inductance between the drivers' supply and the driver ICs themselves. Whenever the ICs turn on, they need to be supplied with a current spike. A local bypass capacitor will supply the fast transient (spike), and the inductance along your 12 V supply trace and its respective ground return trace won't matter much.

d) See intro and a). With BJTs, all hope is lost. With IGBTs or MOSFETs, start by checking the high side drivers' supply voltages (i.e. the voltage across the bootstrap caps).

e) Ideally, you would need zero Ohms. However, the gate capacitance (given you use MOSFETs or IGBTs) together with the source inductance (and the inductance along the gate driving path) form an LC resonant tank. This LC circuit is kept from sustained ringing by making it bad, on purpose, using a gate resistor. 10...100 Ohms may be a good idea. Anything beyond 100 Ohms is usually bad practice. Remember that your drivers need to be able to charge the MOSFETs' or IGBTs' gates fast in order to reduce the switching losses. In your example, 10 kOhms with 1 nF (as a guess for a gate capacitance) yields a time constant of 10 µs. You want your transistors to switch at least 100 times faster than that.