A. Input Stage:____	B. Clamp:____	C. Drain Node Capacitance:____	D. Core Loss:____ , Winding Loss:____
F. Rectifer and Snubber:____	G. Bias Supply:____	H. Current Sense:____	E. Switching Loss:____, Conduction Loss:____

Answers:

a) As the secondary-side losses have decreasted and total losses have remained the same, by definition Z has decreased.
b) Physically this can be interpreted as follows. The secondary-side losses have decreased. Secondary-side losses are incurred during the OFF time of the switch and as such these losses are furnished by the energy discharge from the primary inductance. This means that the primary inductance need not store that much extra energy, and thus, its value can be reduced. In PIXls designs, Z is always set as an input as shown in Figure 1.

Figure 1: Screen capture from the PeakSwitch design spreadsheet showing where the Z factor can be entered.

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Answers:

A) Input Stage — The significant loss here occurs due to the input current that flows while the rectifier diodes are conducting. As the energy loss associated is not stored in the inductor this is a primary-side loss
B) Primary-Side Clamp — The equivalent circuit of the primary-side clamp is the same as that of a secondary winding. The clamp circuit dissipates power during the OFF time of the switch. As such, the energy associated with this loss must be stored in the inductor and thus, it is a secondary-side loss
C) Transformer Node Drain capacitance — Drain-node capacitance loss occurs primarily during turn on event wherein power is being processed by the switch and energy is being stored in the primary inductance. Thus it is a primary-side loss.
D) Transformer Losses :
a. Transformer core loss — Core losses are independent of ON or OFF time. The energy associated with this loss must be stored within the primary inductance. This is therefore a secondary-side loss
b. Transformer Winding loss — Losses associated with the primary winding portion are primary-side losses and losses associated with the secondary-side winding are secondary-side losses.
E) PeakSwitch Losses :
a. PeakSwitch Switching Loss — This is a complicated loss to assign to either primary or secondary-side. The turn-on losses are primary-side losses but the turn-off losses are secondary-side losses. Since turn-off losses are usually more dominant, a conservative approach would be to assign these losses to the secondary-side
b. PeakSwitch Conduction Loss — These losses occur during the ON time of the switch. Thus they are primary-side losses.
F) Output Rectifier and Snubber — Rectifier losses occur during OFF time, while the diode is conducting, and cause the diode snubber capacitor to charge. However actual losses within the snubber occur when the MOSFET turns on, discharging the snubber capacitor. Since the energy in the capacitor was processed during the OFF time though, they are still considered secondary-side losses.
G) Bias Winding Power — The Bias winding looks like a secondary winding, thus it conducts during the OFF time. It is a secondary-side loss.
H) Current Sense — This loss takes place on the secondary side, and is a secondary-side loss.

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Power Supply Application	P, B or S
3 W Cell Phone Charger
25 W Set-top box supply
DC input standby power supply
32 W (81 W peak) inkjet printer supply

Answers:

Part 1

3 W Cell Phone Charger (Secondary Dominated)
The secondary-side CC (Constant Current) sense circuit is usually associated with significant losses (secondary dominated losses). This extra energy must be stored within the primary inductance. The estimated Z factor is 0.7.

25 W Set-Top Box Supply (Balanced)
Here, as the output power has increased, the input stage will no longer use a fusible resistor and the EMI filter inductors will be replaced with a common-mode choke, both of which will reduce the input stage losses. No secondary CC circuits are required so primary and secondary-side losses should roughly balance out. The estimated Z factor is 0.5.

DC Input Standby Power Supply (Secondary Dominated)
The secondary-side losses would slightly dominate in this power supply arrangement because there is no input stage - no rectifier or filter. (In the presence of a rectifier and filter the losses would roughly balance out.)

32 W (81 W Peak) Inkjet Printer Supply (Secondary Dominated)
Usually inkjet printers have a need for high peak-to-average power. Often the output components and transformer-winding wire size are rated for the average power and average thermal rise. However, the instantaneous loss during the peak load condition is high. In addition, such designs may have large primary-to-secondary turns ratios (large VOR). This usually translates into higher secondary RMS currents, which also lead to higher secondary-side losses. The estimated Z factor is 0.6.

Part 2

Verification of Z factor for the design shown

The prototype shown was run at 88 VAC input and at 65 W output (27.74 V, 2.34 A). Total input power was measured as 88.5 W, giving an overall efficiency of 73.3%. That means that the total losses in the power supply, P_LOSS, were 23.59 W.

The following are the prominent areas where losses can be categorized as primary-side losses. All other losses will therefore be secondary-side losses.

1) MOSFET Conduction Losses, P_COND
2) MOSFET Switching Losses (Portion of Turn -Off Losses), P_{SW_PRI}
3) EMI Filter Losses, P_EMI
4) Diode Bridge Conduction Losses, P_DIODE
5) Primary Winding Copper Losses, P_COPPER
6) Input Bulk Capacitor ESR Losses, P_ESR

We need the following parameters to calculate each of the abovementioned losses. Values in parentheses were the measured values for each of the parameters.

(i) Primary RMS Current, I_{SW_RMS} (1.31 A)
(ii) AC Input RMS Current I_{AC_RMS} (1.39 A)
(iii) Bulk Capacitor RMS current I_{CAP_RMS} (1.107 A)
(iv) Average Diode Current, I_{D_AVG} (0.42 A)
(v) MOSFET RDSON, RDS_ON (2.6 Ω)
(vi) Switching Frequency, F_SW (299 kHz)
(vii) Common Mode Inductor Wire Resistance, R_CM (0.6 Ω)
(viii) Transformer Primary Winding Resistance, R_PWDG (0.5 Ω)
(ix) Bulk capacitor ESR, R_ESR (0.7 Ω)
(x) MOSFET Switching Transition Time, ΔT(48 ns)
(xi) MOSFET Switching Voltage, V_SW (269 V)
(xii) MOSFET Switching Current, I_SW (2.42 A)

1) MOSFET Conduction Losses, P_COND

2) MOSFET Switching Losses (Portion of Turn-Off losses), P_{SW_PRI} The MOSFET switching losses are difficult to allocate as primary-side or secondary-side losses. They affect both sides of the transformer and yet they are significant loss elements. We will therefore allocate half the losses to the primary side and the other half to the secondary side.

Figure 2. Waveform shows Drain current (Top) and Drain Voltage (Bottom).

Additionally, it can be observed from Figure 2 that this power supply skips 1 cycle out of every 9 switching cycles to maintain output regulation. We will factor this into the total switching losses

3) EMI Filter Losses, P_EMI Since there are 2 chambers for a common-mode inductor this loss will be incurred two times.

4) Diode Bridge Losses, P_DIODE

5) Primary Winding Copper Losses, P_COPPER

6) Bulk Capacitor ESR Losses, P_ESR

The total primary-side losses, P_{LOSS_PRI} are therefore

P_{LOSS_PRI} = P_COND + P_{SW_PRI} + P_EMI + P_DIODE + P_COPPER + P_ESR
P_{LOSS_PRI} = 10.4868 W

Thus the total secondary-side losses P_{LOSS_SEC} can be written as
P_{LOSS_SEC} = P_LOSS – P_{LOSS_PRI}
P_{LOSS_SEC} = 13.1 W

By definition of the Z factor

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