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Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 4553
Maxim > Design Support > Technical Documents > Application Notes > Circuit Protection > APP 4553
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 4553
Keywords: isolated power, H-bridge drivers, dc-blocking capacitors, isolation capacitors
APPLICATION NOTE 4553
Isolated Power Using Capacitors
Dec 22, 2010
Abstract: An integrated H-bridge driver for isolated power-supply circuits (MAX256) usually drives the
primary of a transformer, but it can also drive a pair of capacitors that substitute for the transformer in
providing isolation and power transfer.
A similar version of this article appeared in the June 15, 2007 issue of EE Times magazine.
Isolated power is usually generated with a transformer, but it can also be generated using capacitors.
For some systems, the constraints of size and cost may favor capacitors.
In Figure 1, the IC (MAX256) is an integrated primary-side controller and H-bridge driver for isolated
power-supply circuits. Its oscillator, protection circuitry, and internal FET drivers usually provide up to 3W
of power to the primary winding of a transformer. In this case, the device drives a pair of capacitors that
substitute for the transformer in providing isolation and power transfer.
Figure 1. This simple circuit generates a capacitively isolated output voltage.
The IC's adjustable switching frequency (100kHz to 1MHz) allows the use of small isolation capacitors,
as illustrated below by an equation giving the capacitor impedance at 1MHz. Losses are negligible at low
output power:
XC = ½πfC = 1/(2 × 3.14 × 10 6 × 0.45 × 10 -6 )
0.35Ω
Complementary square-wave drive signals from the IC (ST1 and ST2) are coupled by the isolation
capacitors and full-wave rectified by the diodes to produce an isolated output voltage. The high switching
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frequency also allows use of a small output capacitor. Ignoring switching losses, the output voltage is:
VOUT = VIN - 2VCAP - 2VDIODE, where VCAP = IOUT × XC. Assuming IOUT = 500mA
VOUT = 5 - 2(0.5 × 0.35) - 2(0.5) = 5 - 0.35 - 1
3.7V
This circuit suits applications for which the potential difference across the isolation barrier is fixed.
(Capacitors provide isolation at dc, but not for ac signals.) With the component values shown and a
500mA load, ripple voltage is about 10% of the dc output level. You can reduce this ripple by increasing
the value of the output capacitor. Other circuit performance includes the power-up response with 8Ω
(~0.5A) load (Figure 2), the no-load power-up response (Figure 3), and the load-transient response
obtained by connecting 8Ω to an unloaded output (Figure 4). (In Figures 2 to 4, Channel 3 is the +5V
supply (VCC), and Channel 4 is the voltage across the output capacitor.)
Figure 2. Power-up response of the Figure 1 circuit with 8Ω load.
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Figure 3. Power-up response of the Figure 1 circuit with no load.
Figure 4. Load-transient response of the Figure 1 circuit, switching from no load to 8Ω load.
Related Parts
MAX256
3W Primary-Side Transformer H-Bridge Driver for
Isolated Supplies
Free Samples Page 3 of 4
More Information
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Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 4553: http://www.maximintegrated.com/an4553
APPLICATION NOTE 4553, AN4553, AN 4553, APP4553, Appnote4553, Appnote 4553
Copyright © by Maxim Integrated Products
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