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P7 - Power Supply

Submitted by tylerjbrooks on Mon, 01/11/2010 - 08:20.

I am starting a new prototype. The new designation is 'Prototype VII' -- or 'p7' for short.

I thought I would try a 65W flyback switch mode power supply. The idea would be to put one of these into each DigiSpeaker. The flyback topology is compact, low part count and cheap while being relatively high in efficiency and performance.

Click here for the schematic.
Click here or here for the BOM. Q1 price is $22.35. Q1K price is $9.90. Both prices are without transformer, various connectors and so on.

The Design (procedure taken from the Power Supply Cookbook, Marty Brown):

Input 90VAC to 240VAC @ 50/60Hz
Output Three Outputs:

  1. Output1: +20VDC @2.25A, Ripple: 100mVpp, Regulation: +-5%
  2. Output2: +15VDC @.333A, Ripple: 100mVpp, Regulation: +-10%
  3. Output3: +20VDC @3.00A, Ripple: 100mVpp, Regulation: +-10%
Total Output Power (20V*2.25A) + (15V*.333A) + (5V*3A) = 65W
Total Input Power
Flyback topologies tyically get 80% efficiency. 		65W / 0.8 = 81.25W
DC Input 110VAC -> Vlow = 90 * 1.414 = 127VDC
110VAC -> Vhigh = 130 * 1.414 = 184VDC
220VAC -> Vlow = 185 * 1.414 = 262VDC
220VAC -> Vhigh = 240 * 1.414 = 340VDC
Average Input Current Iin_high = 81.25W / 127VDC = 0.639A
Iin_low = 81.25W / 340VDC = 0.239A
Peak Current Ipeak = 5.5(65W) / 127VDC = 2.815A
Heat
MOSFETs typically have 35% of the losses.
Rectifiers typically have 60% of the losses. 		Total Loss = 81.25W - 65W = 16.25W
MOSFET Loss = 16.25W * 0.35 = 5.68W
20V Rectifier Loss = (45/65W) * 16.25 * 0.6 = 6.75W
15V Rectifier Loss = (5/65W) * 16.25 * 0.6 = 0.75W
5V Rectifier Loss = (15/65W) * 16.25 * 0.6 = 2.25W
Transformer Primary Inductange = (127VDC * 0.5) / (2.815A * 50KHz) = 452uH
Core Gap = (0.4 * PI * .452mH * 2.815A * 10**8) / (0.904 * 2000**2) = 0.044cm (or 17mils)
Core Selection: Magnetics, Inc. 0F43007EC and 0F43007G044
Primary Turns = 1000 * (.452/100)**0.5 ~= 67Turns
20V Secondary Turns = (67 * (20 + 0.5) * (1 - 0.5)) / (127 * .5) ~= 11Turns
15V Secondary Turns = (15 + 0.9)(11Turns) / 20.5 ~= 9Turns
5V Secondary Turns = (5 + 0.9)(11Turns) / 20.5 ~= 3Turns
Output Filter 20V Reverse Voltage = 20V + (11T/67T)*340VDC = 75.8V @ 2.25A -> MUR420
15V Reverse Voltage = 15V + (9T/67T)*340VDC = 60.7V @ 0.333A -> MUR120
5V Reverse Voltage = 5V + (3T/67T)*340VDC = 20.2V @ 3.0A -> MUR420
20V Output Capacitor = (2.25A * 18uS) / 100mVpp = 405uF -> 2x 220uF @ 35V
15V Output Capacitor = (0.333A * 18uS) / 100mVpp = 60uF -> 1x 100uF @ 25V
5V Output Capacitor = (3.0A* 18uS) / 100mVpp = 270uF -> 1x 220uF @ 10V
Power MOSFET Vdss > 340 + (67/3)(5+0.5) = 462V
Ipeak < 3A
Select: STMicro STP4NK50ZD
Feedback Regulation Start by assuming a 1mA regulation current per volt.
R1 = 5V/5mA = 1Kohm
R2 = 5 - (2.5 + 1.4) / 6mA = 180ohm
R3 = 2.5V / 1mA ~= 2.7Kohm
Isense = 2.5V / 2.7Kohm = 0.926mA
Spread the regulation between all three outputs. 20V and 15V get 40% of the regulation each. 5V get 20% of the regulation.
R4 = (5V - 2.5V) / 0.2(0.926mA) = 13.5Kohm
R5 = (15V - 2.5V) / 0.4(0.926mA) = 33.75Kohm
R6 = (20V - 2.5V) / 0.4(0.926mA) = 47.25Kohm
Current Sense Rcs = Vcs / Ipeak = 0.7V / 2.815A = 0.249ohms @ 2.4W
Feedback Loop 20V Pole = 1 / (2 * PI * (20/0.6) * 440uF) = 10.85Hz
15V Pole = 1 / (2 * PI * (15/0.1) * 100uF) = 10.61Hz
5V Pole = 1 / (2 * PI * (5/1.0) * 220uF) = 144Hz
ADC = ((340 - 5)**2 * 3T) / (340 * 67T) = 14.78
GDC = 20log(14.78) = 23.4dB
Gxo = 20log(10KHz/10.85Hz) - 23.4 = 35.89db or 62.31
So...
C1 = 1 / (2 * PI * 13.5Kohm * 62 * 20KHz) = 9.5pF
R2 = 13.5Kohms * 62 = 840Kohms
C2 = 1 / 2 * PI * 10.85Hz * 840Kohms = 17.4nF
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