TN0024 STMICROELECTRONICS | Alldatasheet

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Technical content

A warning signal at a time period is often requested from a power supply for the load to complete housekeeping chores before the output voltage drops out of regulation. A circuit to monitor AC input voltage and a bulk capacitor of sufficient size are often used to meet these requirements. The HOLD-UP time of an off line, high frequency power supply can be defined as the time required for the output voltage to remain within regulation after the AC input voltage is removed. It is commonly expressed in ms from a specific input voltage, which is usually less than the nominal AC input voltage, and at a specific output power. The power supply is designed to regulate output voltage at the DC bulk voltage which is reached after the HOLD- UP time. If a HOLD-UP time is required, there are tradeoffs with respect to the power supply design input voltage and regarding the size of bulk capacitors. Often the major part of the power supply design, on the primary side, depends on the lowest DC bulk voltage after the HOLD- UP time in which the power supply can operate. This document presents a comparison between lab data, P-Spice simulation and MathCAD analysis of the same high frequency off line power supply. The power supply is a VIPer53DIP-E demo board with a universal 85 to 264 VAC input voltage and a 12 V output voltage with a 2 A load. The inrush resistor, R1, is 3 Ω and the common mode inductor, L1, is about 2.5 Ω. The bulk capacitor, C2, is 68 µF and measures about 60 µF. T h e requirement is for a 10ms HOLD-UP time, an AC voltage at turn off of 110 V AC and the power supply is designed to operate at an input voltage of 80 VDC.

Equation derivation TN0024

1 Equation derivation

The equation derivation approach is to determine the minimum bulk voltage with energy equations and then use the energy at this voltage to determine the minimum operating voltage. The symbol for overall efficiency is cc and the efficiency used when the AC line is removed is represented by No.

2 MathCAD

MathCAD is used to determine the minimum operating voltage and also to determine the bulk capacitor value for a HOLD-UP time, T up, of 10 ms. Figure 1 shows the AC input voltage as a blue dashed line and the equivalent DC input voltage in red. Note that for a bulk capacitor of 60 µF (x-axis), the DC voltage is about 80 V (y-axis). A procedure to calculate the bulk DC voltage or the bulk capacitance is given in Section 6.1., Equation 27 and Equation 28.

3 P-SPICE

A P-Spice simulation of a bulk capacitor discharge can be approximated using a voltage controlled current source G1 (see Figure 2). Using an effective voltage range from the peak bulk voltage to a minimum operating voltage, a load current can be simulated which is inversely proportional to the bulk DC voltage. For an example, bulk voltages of 80 to 150 V DC can be monitored from a 110 VAC input voltage turn off with a 10ms HOLD - UP time requirement and a power supply output power of 24 W. The following equations calculate the bulk capacitor current supplied to the power supply at 150 V and 80V for an output power of 24 W and an efficiency of 87%: Equation 1 Equation 2 Equation 3 Equation 4 A voltage controlled current source with a 230 V reference can be used with a gain of 0.0023 to simulate the above currents. Po 24W= N0 . 8 7= Pin Po Vc 150V= IC 27.6 Vc 80V= IC 27.6 µ

0.184 A when the bulk voltage is 150 V and 0.345 A when the bulk voltage is 80 V.

4 Lab data

5 Equation derivation

  • Energy equation for a capacitor (C): Equation 5 Equation 6
  • How to calculate the energy in C for each half line cycle: Equation 7 Equation 8

Table 1. Current simulation

2 Vbmin

2 Ein

Equation derivation TN0024

  • How to calculate the power supply input energy: Equation 9 Equation 10 Equation 11
  • How to calculate the peak bulk voltage: Equation 12 Equation 13
  • How to calculate the bulk energy at low bulk voltage minus the load energy for HOLD- UP time Tup: Equation 14 Equation 15 Equation 16 Equation 17
  • How to calculate minimum DC bulk voltage: Equation 18 Ein Pin Pin Po Ein Po Vbpk Vdcoff Vd– Rin Po NVdc off Vbmin Vdcoff Vd– Rin Po NVdc off ⎛⎞ 2 Po Ebulk off Ebulk on Eload–= Ebulk on CV bmin Eload PoTup No Ebulk off CV bmin No Vdcmin 2Ebulk off

TN0024 Equation derivation Equation 19 Equation 20

  • How to factor out Po/C: Equation 21 Equation 22 Equation 23
  • Solving for C in Equation 23 : Equation 24 Vdcmin Vbmin 2 2PoTup CNo Vdcmin Vdcoff Vd– Rin Po NVdcoff ⎛⎞ 2 Po CNo Vdcmin Vdcoff Vd– Rin Po NVdcoff 2 Po ⎛– ⎠ ⎞ 1 No ⎛⎞= Vdcoff Vacoff 2•= Vdcmin Vacoff 2• Vd– Rin Po NVacoff ⎛⎞ 2 Po No ⎛⎞–= C Po No Vacoff 2• Vd– Rin Po NVacoff ⎛⎞ 2 Vdcmin

6 HOLD-UP graph: T up = 10 ms

Figure 1. Minimum operating voltage vs. bulk capacitance

6.1 HOLD-UP example

  • = 110 AC voltage at turn-off
  • Po = 24 Output power
  • η = 0.84 Efficiency running
  • No = 0.87 Efficiency at turn-off
  • C = Bulk capacitor
  • f = 60 Line frequency
  • Tup = Desired HOLD-UP time
  • Vd = 1.2 Voltage drop of the input diodes
  • Rin = 5.5 Inrush resistor and EMI filter resistance in the AC line Vdcmin c() Vacoff 2V d– Rin Po• Vacoff 2N•• ⎛⎞ 2 Po No ⎛⎞•–= Vacmin c() Vdcmin c() 50 52 54 56 58 60 62 64 66 68 70 72 74 7 640 100 Vdcmin c() Vacmin c() c1 06⋅ Capacitance (µF) Red solid trace: Minimum DC voltage Blue dash trace: Minimum AC voltage Vacoff 60 10 6–• 10 10 3–•

Figure 4. VIPer53 power supply schematic

22 C13

2 X 35mH

Figure 5. Measured voltage vs. time

7 VIPer53DIP-E (see Figure 4)

TOVL controlled by an external capacitor.

8 Revision history

Table 2. Revision history