ZXGD3101EV2 EVALUATION BOARD USER GUIDE
Description
This document describes how to connect and evaluate the ZXGD3101EV2 evaluation board, Figure 1.
The purpose of this board is to demonstrate synchronous rectification and driving of a MOSFET as a
Schottky/ultra-fast recovery diode replacement in Flyback converters. When the board is used to drive
a synchronous MOSFET, it will yield efficiency improvement, whilst maintaining design simplicity and
incurring minimal component count. The ZXGD3101 senses the voltage across the MOSFET and
generates the gate drive voltage when a negative voltage is detected across the drain-source pin.
This evaluation guide also includes useful guidelines on conditioning of Flyback converters, as well as
a design tip to overcome premature driver turn-off and maximize the effectiveness of synchronous
rectification.
Note: The evaluation board is not recommended to be used with Flyback converters above 100kHz
switching frequency.
Figure 1 Evaluation board layout and connection diagram
Reference design
The ZXGD3101EV2 is configured to the reference design in Figure 2. The target application for the
device is external adapters where the typical output voltage ranges from 12V to 20V. However, the
evaluation board will work with output voltages up to 40V.
Power, which could be sourced directly from the output of the power supply, is applied to the terminal
block P1. At the other end of the board is a location for a three way header, P2. This is not fitted, so
as to allow flexibility of mounting (forward or reverse). The purpose of the header is to allow the board
to be soldered directly across a TO220 packaged synchronous MOSFET.
R4, Q2, D1 and C1 are configured as a simple series regulator to maintain a stable Vcc. The values of
Rref and Rbias in Figure 2 are based on a 10.3V Vcc. For supply voltages below this, these resistor
values need to be reduced proportionally to maintain 5.3mA and 3.2mA into the ‘REF’ and ‘BIAS’ pins
respectively. Refer to the datasheet for further information on selection of the resistor value.
A fixed-bias-point constant current source (Q1, R2, R1//R5 and R3) is included on the board. The
current source is not active in the original board setting and is only intended for elimination of
premature turn-off problem on the controller (refer to the section ‘Overcoming premature turn-off’ for
details).
Issue 1 – December 2009
© Diodes Incorporated, 2009
www.diodes.com
ZXGD3101EV2
Figure 2: Evaluation board schematic diagram and connection
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: Evaluation board component details (BOM)
Ref.
U1
Q1
Q2
D1
C1
R1
R2
R3
R4
Rbias
Rgate
Rref
P1
P2
Value
ZXGD3101
FMMTA92
FMMT491A
11V Zener
1uF 50V
8.2k
1K
100
10k
1.8k
0R
3K
2-way terminal
3-way header
Package
SO8
SOT23
SOT23
SOT23
1206
0805
0805
0805
0805
1206
1206
1206
Part number
ZXGD3101N8
FMMTA92
FMMT491A
BZX84C11
C1206X105K5RAC
Generic
Generic
Generic
Generic
Generic
Generic
Generic
Generic
Generic
Manufacturer
Diodes Zetex
Diodes Zetex
Diodes Zetex
Diodes Inc.
Kemet
Notes
500mW
300mW
X7R 10%
125mW, 1%, 200ppm/ºC
125mW, 1%, 200ppm/ºC
125mW, 1%, 200ppm/ºC
125mW, 1%, 200ppm/ºC
125mW, 5%, 200ppm/ºC
125mW, 5%, 200ppm/ºC
125mW, 5%, 200ppm/ºC
Issue 1 – December 2009
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2
ZXGD3101EV2
Evaluation procedure and operation
To perform a quick functional test of the ZXGD3101, the evaluation board can be used to drive a
MOSFET as a diode replacement in high-side-rectification (see Fig. 3a), as the board can float to any
potential. In practice, the supply voltage could be derived from an auxiliary supply winding across the
transformer secondary. If the board is used for comparison against an existing synchronous
rectification solution, the existing controller must disabled before proceed with the testing.
The recommended device implementation is low side synchronous rectification (Fig. 3b), due to the
ease of acquiring the required supply voltage directly, either from the power supply output post
bleeder resistor, or from the emitter-follower-configured transistor. Before doing this test, it is
important that the existing diode has been removed and/or a short has been applied across its
cathode and anode terminals. The track linking the negative terminal of the converter’s output
capacitor to the transformer secondary-side output should then be cut, and a MOSFET should be
inserted. In general, the MOSFET should be selected to drop between 50 to 150mV at the peak of the
secondary-side current to ensure MOSFET enhancement. The breakdown voltage of the MOSFET
must be higher than the maximum drain-source voltage stress, plus some margin. Designers
interested in squeezing the last percent of efficiency out of the module can place an additional
Schottky or Ultra-fast-recovery diode in parallel with MOSET. The diode prevents body-diode
conduction, so the trace inductance between it and the MOSFET should be kept small to create an
efficient circulating energy flow path.
Figure 3 Test options for ZXGD3101EV1 a) high side and b) low side
(a)
(b)
Figure 3: Test options for ZXGD3101EV2
a) high side and b) low side
To check for functionality, the circuit waveforms should be probed using an oscilloscope probe with a
minimal length for the ground pin, and the probe should be connected directly to the pins of the
device. If a current probe or transformer is used to measure reverse current flow, excessive wire-loop-
inductance and injection of noise, which could disturb normal functioning of the controller, should be
avoided.
At synchronous MOSFET turn-on, current starts to flow through the body-diode after the primary
switch turn-off (see Fig. 4). When this occurs, the drain of the MOSFET will be around -1.25V with
respect to ground, due to body-diode conduction. The detector stage within the ZXGD3101
determines when the MOSFET needs to turn on by measuring the change in polarity of the V
SD
differential voltage, which, in turn, determines when the current is flowing through the secondary side.
The turn-off phase of the ZXGD3101 happens differently depending on the mode of operation. It
should be noted that the device is most suited to discontinuous and critical conduction mode, however
Issue 1 – December 2009
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ZXGD3101EV2
it can also be used in continuous conduction mode. In discontinuous and critical conduction mode, the
MOSFET current decays linearly and the controller proportionately backs off its gate-drive output
when the on-resistance-induced conduction voltage drop is less than -50mV. Upon the conduction
voltage crossing the turning off threshold, the gate drive is turned off quickly to eliminate any reverse
current flow (see Figure 4a). If the board is evaluated with a converter in continuous conduction mode,
the secondary side circulating current does not decay to zero prior to primary MOSFET turn-on. The
controller then turns off the MOSFET quickly when the primary MOSFET current starts rising in less
than 50ns delay time, as shown in Fig. 4b, so the possibility of cross conduction is minimized. This is
critical because cross conduction due to the primary side MOSFET and secondary rectification
MOSFET conducting simultaneously will degrade efficiency due to the nature of the fast transition.
(a)
(b)
Figure 4: Synchronous rectification operating waveforms (a) Critical conduction mode and (b)
Continuous conduction mode
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ZXGD3101EV2
Conditioning the power supply to maximize efficiency of ZXGD3101
The ZXGD3101 can be susceptible to noise if a proper snubbing circuit on the primary side is not
devised. Any high-frequency-resonance-ringing on the drain of the primary MOSFET will be reflected
across the transformer as multiple synchronous MOSFET V
SD
transitions, which will cause spurious
turn-on to occur. The controller is then not able to fully enhance the MOSFET until the oscillation is
stabilized. To prevent this problem, the user is advised to strengthen the primary switch snubber
circuit through either a damping resistor R
d
(see Fig. 5) or alternatively an additional snubbing R-C
network. These have the effect of eliminating the oscillation by limiting the peak D
clamp
reverse
recovery current and soften its reverse recovery characteristic. The improvement on the rising edge of
the gate drive can be observed as in Figure 6.
Figure 5: Recommended design for a synchronous rectified Flyback converter
Another snubber network comprising of R
snub
and C
snub
should be fitted across the synchronous
MOSFET to dampen out high frequency oscillations at the MOSFET’s fast turn-off edge. If the
amplitude of oscillations is high, then the drain voltage could ring below the turn-on threshold. The
controller could then be falsely triggered and provide an output high to drive the MOSFET gate. Apart
from preventing premature turn-on of the controller, this also has the added benefit of reducing
conducted EMI generation and device voltage stress.
Furthermore, any parasitic inductance due to a combination of printed circuit board traces and
component leads can also cause the voltage at the drain input of the ZXGD3101 to ring about ground.
Proper layout attention must be paid to ensure the integrity of the V
SD
differential voltage. To mitigate
noise induced malfunction, it is important to keep the drain input on the controller as close as possible
to the synchronous MOSFET, preferably within 10mm. A minimal gate drive loop will also negate the
effect of loop inductance inducing oscillation to the controller’s output gate drive voltage, reducing the
requirement for series gate resistor damping.
Issue 1 – December 2009
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5
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