ZXGD3104EV2 USER GUIDE
Introduction
The purpose of this board is to demonstrate the driving of a synchronous MOSFET as a Schottky
replacement in isolated power supplies. The circuit is suitable for use in AC/DC Flyback converters.
End applications include external power adaptors for laptops greater than 90W, and LED monitors.
When used to drive a low on-resistance MOSFET, the board increases power efficiency whilst still
maintaining simplicity of design.
Performance
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•
•
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Recommended supply voltage VIN: 19V
Gate voltage clamped to 12V maximum
Ideal for Quasi Resonant operation
Switching frequency up to 250KHz
Ordering Information
Order Number
ZXGD3104EV2
Caution: Do not connect the evaluation board to a
supply voltage, VIN, greater than 25V!
Figure 1. Evaluation board layout
Issue 1 – October, 2011
© Diodes Incorporated, 2011
www.diodes.com
ZXGD3104EV2
Evaluation guide
There are two possible test setups for the evaluation board. The preferred setup is low side
synchronous rectification (see Figure 2b), due to the ease of acquiring the supply voltage to the board
directly from the output of the power supply. The other option is shown in Figure 3a. Figure 3a shows
the board driving a MOSFET for high side rectification.
Low side synchronous rectification
1. Remove the original Schottky from the power supply
2. Apply a short across the Schottky’s K and A terminals
3. Cut the track that connects the negative terminal of the output filter capacitor to the output of
the transformer winding.
4. Insert a low on-resistance MOSFET between the cut tracks. The Drain terminal of the
MOSFET should be connected to the output of the transformer winding, whilst the Source
terminal is connected to the output capacitor.
Caution: The MOSFET breakdown must be higher than the maximum Drain-Source voltage
spike, plus a 10% to 30% margin.
5. Connect the power supply’s output voltage, 19V, to the terminal block P1 (see Figure 2b).
6. Connect an AC voltage source to the power supply’s input.
7. Turn on the AC voltage source and measure the efficiency.
High side synchronous rectification
8. Remove the original Schottky from the power supply
9. Insert a low on-resistance MOSFET to replace the Schottky. The Source terminal of the
MOSFET should be connected to the output of the transformer winding, whilst the Drain
terminal should be connected to the output capacitor
10. Connect a 10V auxiliary supply to terminal block P1 (see Figure 2a).
11. Connect an AC voltage source voltage to the power supply’s input.
12. Turn on the power supply and measure the efficiency.
(a)
(b)
Figure 2: Test options for evaluation board a) high side rectification and b) low side
rectification
Issue 1 – October, 2011
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ZXGD3104EV2
Conditioning the power supply to maximize efficiency
Any stray inductance in the load current path may cause distortion of the drain-to-source voltage
waveform, leading to premature turn-off of the synchronous MOSFET. In order to avoid this issue,
drain voltage sensing should be done as physically close to the drain terminals as possible. The PCB
track length between the Drain pin and the MOSFET’s terminal should be kept to less than 10mm.
MOSFET packages with a low internal wire-bond inductance are preferred for high switching
frequency power conversion, to minimize body-diode conduction.
After the primary MOSFET turns off, its drain voltage oscillates due to the reverse recovery of the
snubber diode. These high frequency oscillations are reflected into the transformer secondary winding
and the drain terminal of the synchronous MOSFET. The synchronous IC senses the drain voltage
ringing, causing its gate output voltage to oscillate. The synchronous MOSFET cannot be fully
enhanced until the drain voltage stabilizes.
In order to prevent this issue, the oscillations on the primary MOSFET can be damped with either a
series resistor Rd to the snubber diode Dsnub, or an R-C network across Dsnub as shown in Figure
3. Both methods reduce the oscillations by softening the snubber diode’s reverse recovery
characteristic.
Figure 3: Techniques to prevent/reduce gate voltage oscillations
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ZXGD3104EV2
Waveform and efficiency measurements
The operating waveforms of the controller can be measured using oscilloscope probes. If a current
probe or transformer is also used to measure the MOSFET current, the wire length should be kept
short to avoid excessive loop-inductance which could disturb the controller operation.
Figure 4 shows the operating waveforms in the Flyback converter inside a 19V 90W output power
adaptor. The power adaptor also incorporates a boost power factor correction stage to comply with
IEC61000-3-2.
The controller senses the forward voltage drop of the parasitic diode within the MOSFET, and when
the diode is in conduction, applies a voltage to the MOSFET gate, turning on the MOSFET after an
initial delay time. The gate output voltage of the controller is then proportional to the sensed voltage.
In QR mode, the gate voltage is reduced as the MOSFET’s Drain current decreases. This ensures
that the MOSFET is turned off at the zero current point, with little or no reverse current. Another
advantage is that this technique prevents early termination of the gate voltage at low Drain current.
Early termination of gate voltage can reduce the efficiency due to body-diode conduction loss.
Figure 5 shows that 90% efficiency can be achieved in the 90W adaptor when using the evaluation
board to drive a 10m 100V synchronous MOSFET.
Figure 4: Operating waveforms in QR mode with a 10m
100V synchronous MOSFET
Figure 5: Efficiency measurement in a 19V 90W power adaptor
Issue 1 – October, 2011
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ZXGD3104EV2
Board schematic
Figure 6 shows the circuit schematic of ZXGD3104EV2. Power for the controller is applied to the
terminal block P1. A three way header, P2 is located at the other end of the board. The header allows
the board to be soldered directly across a synchronous MOSFET in a TO220 package. The board can
also be used with an SMD MOSFET by connecting the pin-outs accordingly.
The evaluation board is designed to accept a supply voltage of 19V, which is the typical output
voltage of a laptop power adaptor. If the power supply’s output voltage is required to be greater than
25V, a voltage regulator should be used to step down the high voltage to 19V before connecting to the
evaluation board. The values of the threshold setting resistors, Rref and Rbias, are chosen for
Vcc=19V. Please refer to the ZXGD3105N8 datasheet for more information.
DZ is an optional Zener diode connected to the BIAS pin of the controller to limit the maximum gate
voltage to 12V. DZ is recommended when the controller is used to drive a high-gate-charge
synchronous MOSFET or/and at high switching frequency. This configuration reduces the gate-charge
switching loss.
Figure 6: Circuit diagram
Please note that the component part numbers are given as a guide only. Due to continual component
development, all parts quoted should be checked for suitability and availability with their respective
manufacturers.
Table 1: Part list
Ref.
U1
DZ
C1
P1
P2
Rbias
Rgate
Rref
Value
25V VCC max
synchronous controller
12V Zener
1uF 50V capacitor
2-way terminal
3-way header
6k2 resistor
0R resistor
8k2 resistor
Package
SO8
SOT23
1206
Part number
ZXGD3104N8
BZX84C12
C1206X105K5R
Manufacturer
Diodes Inc.
Diodes Inc.
Kemet
Notes
X7R
generic
generic
1206
1206
1206
Generic
Generic
Generic
1%, 200ppm/ºC
5%, 200ppm/ºC
1%, 200ppm/ºC
Issue 1 – October, 2011
© Diodes Incorporated 2011
www.diodes.com
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