Bob's DIY Fun

Alternate Output Voltages

 

 

Higher Voltage Operation

The PSU2.2 circuit was designed to be scalable to provide output voltages other than 15V.  The following section describes the changes required to make a 65V regulated supply and provides guidelines for calculating changes for other voltages greater than 15V.[1][1]

 

The obvious place to start engineering is the voltage ratings of the various components.  Almost all of the capacitors need to be changed from the original BOM.  A Bill of Materials is provided as Appendix B and includes Digikey and Mouser part numbers.

 

The main filter capacitors (C4, 5, 27, 28) are 16 mm diameter with 10 mm lead spacing.  Since we are shooting for 65V output, use at least 80V rated capacitors.  The highest value capacitors that I found to fit are 470 µF 100V Panasonic FC, although other less expensive capacitors that fit the space allocated could be used.  Since the total capacitance of the main filter is only 940 µF, output current will be limited to avoid excessive ripple. 

 

When I wanted to build my test regulator, Digikey was out of stock of the 470 µF Panasonic caps, with delivery time undetermined.  Have some 1000 µF 100V Nichicons for another project that are 18 mm diameter with 7.5 mm lead spacing.  There is a little interference cap to cap but not with any other parts.  Zip tie them together and they’ll sit straight.  It all depends on you tolerance for not quite fully seated mounting.  The testing that follows was completed with two of these caps installed.

 

Similarly, the output capacitors (C10, 11, 20, 21) need to be upgraded to 80+V types. The highest value capacitors that I found to fit are 100 µF 100V Panasonic FC.  Again, other types can be used, although I was unable to find any higher value.

 

The large film capacitors also need to be uprated, except for C14 and C17 which will only see 10V.  The highest value that fits the 5 x 7.5 mm space allocated is 0.47 µF, although a 1 µF capacitor with a 7.5 x 7.5 mm footprint is available.  It fits all positions as long as you are willing to give up the trim pot.  I used the 0.47 µF caps for my initial tests. 

 

The BC546C and BC556C transistors specified in the baseline version are rated at 65V.  That’s a bit too close for comfort, so use MPSA42 and MPSA92 instead.  These devices have much lower current gain than the BCs specified for the original circuit, so expect somewhat lower performance with them.  Stick with the BCs up to 45-50V for best performance. 

 

WHEN USING MPSA42/92 INSERT THEM OPPOSITE THE ORIENTATION SHOWN ON THE SILK SCREEN

 

Now we go through the circuit to verify that all parts are operating within their current and power limits.

 

Starting with the voltage reference current we find that the LM4040-10 is specified for operation from 120 µA to 15 mA.  There is no data offered on noise performance or regulation vs. current.  Choose R4 and R19 to keep the current within these limits.  The 15V version has 5 mA in the reference.  To calculate the value of R4 and R19 use:

 

R4= (Vout-Vref)/Iref

 

 


Using our LM4040-10 for Vref, 5 mA and 65 V target output, we come up with 11K for R4.  The dissipation in R4 is 55V x 5 mA = 0.275W.  Therefore you’ll need to use a 1/2W resistor.   I had some 10K 1/2W resistors handy, so I used them, increasing the reference current to 5.5mA.  They don’t quite fit in the space provided, so you will have to mount them higher than the surrounding resistors.  This is probably a good idea anyway, given the dissipation. 

 

Another option is to reduce the current in the reference.  If we use 22K for R4 and R19, we get a reference current of 2.5 mA and 0.14W dissipation.  This is still plenty of current for the LM4040, but if you use a zener reference, it may be a bit noisier than at 5 mA. 

 

Now choose R3 and R18.  These should be 10 times the value of R4 and R19 to maintain the short circuit protection.  Since we used 22K for R4, use 220K here.  The current and power dissipation in these parts is so low that they can be 1/8W. 

 

Next we’ll look at the dissipation in the differential pair.  The MPSA42 is rated at 0.6W.  R10 and R12 set the current in the differential pair.  The voltage across them is Vref - .6V (the Emitter to Base voltage of the differential transistors), therefore the current is:

 

I = (Vref - .6)/R10

 

With R10=1K, we have 9.4 mA in the differential.  In steady state operation the voltage across R1 must be 1.2V, since it is the same as the two Emitter to Base voltages in the pass devices.  Therefore the current in the T3 leg is 1.2 mA, leaving 8.2 mA in the T4 leg. 

 

Since I plan to use a 55V transformer to drive this regulator, the filtered DC should be around 76V.  The voltage across T3 is.  This makes the dissipation in T3 .077W, well within its limits. 

 

 Turning to T4, the voltage across it is 65V - Vref  + .6 = 55.6V.  This makes its dissipation 0.46W.  This is within its rated dissipation, but staying under half its rated dissipation will help ensure longer life, especially when the regulator is inside a hot amplifier case. 

 

If R10 and R12 are increased to 2K, the current in the differential pair drops to 4.7 mA.  T3 will still pass 1.2 mA, so the current in T4 is 3.5 mA.  Now the T4 dissipation is only 0.192W.  Now I feel more comfortable, so 2K it is. 

 

The feedback network needs to be changed to get the proper output voltage.  You could reduce R11 or turn P1 down, but the dissipation in R2 would be 3W.  As discussed when determining the value of R4, somewhere in the neighborhood of 20K will allow the use of 1/4W resistors.  To determine the value of R2 use the following equation:

 

R2 = (Vout/Vref -1) x R11

 

If we use R11 = 3.3K then R2 is 18150.  The nearest standard value is 18.2 K which also happens to be in my parts bin.  The power dissipation is 0.17W, safe enough for a 1/4W resistor.  You could increase them, keeping the ratio the same and reduce the dissipation further. See the spreadsheet to calculate values.

 

The last parts to consider are R6 and R17.  We need to increase them to limit the current drawn by the LEDs as well as their power dissipation.  Left at 3.3K the LEDs would receive 19 mA and R6 would dissipate 1.2W.  

 

Just a few mA are needed to give a power indication, although most LEDs can handle 20-30 mA.  For the sake of argument, I have decided that 4.2 mA makes the LEDs bright enough.   To find the required resistor:

 

R6 = (Vout - Vled) / Iled

 

Most green LEDs drop 2V, reds 1.2V and blues 5.  Plugging green into the above equation, we get 15K for R6.   This gives us .26W dissipation, so a 1/2 W resistor is required.  The leads will fit in the board, but the resistor must be mounted slightly above the board as the resistor length is too long for the lead spacing.

 

So, how much current can this regulator deliver?  As explained in the thread, the pass transistor can dissipate about 5W if the 2” heat sink is used.  In my case, using 77 volts unregulated and 65 volts regulated, I get a power dissipation limited current of 417 mA.  Modeling the supply with PSU Designer from Duncan’s Amps shows that we should expect ~2.8V of ripple at 400 mA output and that the unregulated voltage stays high enough allow proper regulator operation.  However, testing is required to see how much current the regulator can deliver with acceptable performance.