Ive built alot of h-bridge circuits from the net but I would like to be able to understand the process of designing my own.

Design specifications
Im wanting to use tip 120 darlington NpN transistors for the bridge.
Im driving them with a pic so am limited to 5V and 20 mA
The motor is rated for 1.5A at 5V but runs fine off 6V

Seemed logical to me to start from the bridge first so here goes.
Since the Tip102's have a gain of 1000 (confirmed with a test circuit) each base should only require 1.5 mA but since its being used as a switch I should drive it into saturation correct? 120mA is its saturation point, assuming a 5V source that would be roughly 40 ohms of resistance. The part I dont understand is that typically the low side transistor has a higher resistance value then the high side even though they use the same transistors, why is that?

Since the tip120's drop aprox. 1.4 Volts from collector to emitter I should have a source for the motor thats 2.8V higher then required correct? If so 8-9V should be fine IMO.

Anyhow assuming the above is correct I cant directly drive the transistors with my pics output so I would use a 2N2222 NpN transistor to amplify its current output. My NpN has a gain of 129 so its base current should be roughly 1.9mA to provide enough current to both bases of the darlington transistors to achieve saturation (240mA for both). 5V/1.9mA = 2631 so around 2.5K should be used as a current limiter on the NpN's base correct?

Now for the wiring part

Since the base requires atleast .7Volts should I drive the base of the 2N2222 directly from the pic or build a voltage divider network? Most of my schematics drive it directly but how do you figure the resistance values then? The voltage drop across the resistor when driven directly is extremely low, I would think it should be close to 5V but its been more like 300mV. Also should the NpN's emitter go dirctly to the tip120's bases or should I build a voltage divider network.

Thanks in advance, I appreciate any corrections or additions that will help me understand how to design transistor circuits.

 

you can't really use npns in the high side of the bridge*. consider pnps there.

j.

*you can, but it's not easy.

 

Yup n-type/npn on teh top is a bit didgy. If you thing about it in a H-bridge the emitter of teh top two will be jumping all over the place.

Since your DC-link is of low voltage (ie comparable to your control signal) P type is you best solution (you dont ahve to worry abt the extra logic inverstion as well).

When the Dc-link exceeds the control voltage available what you ahve to do is provide an isolated control and power to the top channels. That way the local emitter of the top switches are furm referenced against an isolated supply so it doesnt matter what the emitter of that tranny does

 

hfe is dependant on collector current (graph in datasheet) it varies with load.

No, 120mA is the maximum base current. When a transistor reaches saturation depends on the load. if your motor requires, for example, 1A then it'll already be saturated with few mA's of base current (it's a darlinton for something )

Don't really know what you mean here.

Depends, again, on the load. According to my datasheet Vce is only 0.6 volt at Ic = 1A. But indeed gets higher with larger load.

The pic will be able to drive the Tip's directly without buffering with a second transistor, they're darlinton's, they have huge gain, therefore not requiring an extra buffer. but, like suggested by the others you probably want to use the Tip's PNP counterpart in the high side of the bridge, in wich case you will need another transistor to invert the logic.

You drive the base trough a resistor in series with it. To calculate the resistor you need to know Vbe for your situation (depends on load). subtract this from the voltage you use to drive the resistor (5V from the pic?) this will be the drop across the resistor.
you then use ohms law to calculate the resistance required fore base current you want.

In short... everything depends on the load.
How much current does your motor require anyway?.

 

Max current is 1.5A but 1 amp is typical at 90% PWM.

I attached an example circuit from "Pic Microcontroller Project Book" by John Iovine. Its only half of the bridge but shows what I was refering too as the low side having a higher resistance in the current limiter then the high side, unless Im confusing the operation of the circuit. Both transistors used are 120Tips.

Attached Images

transistor_905.jpg (85.8 KB, 230 views)

 

In this circuit the lower transistor is used as a switch, the top one is placed as emitter follower. They're diffirent usage of the transistor, that's why the base resistance isn't the same.

The top transistor won't switch, it's emitter will follow base voltage (minus Vbe). As a result it will also dissipate more heat.

If you want both the top and bottom of the bridge to switch, then you need a PNP at the top.

 

Thanks Exo that's exactly the answers Ive been looking for.