Datasheet

Scope Volt-meter Volt-meter Current-meter

85 Volt, 5 Amp

Power Supply

with Current

Meter

200 Watt, 60

Amp Electronic

Load

Evaluation Board

ON/OFF

(SHUTDOWN)

OUT

Jumper

Digital Isolator

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Figure 2. Typical Evaluation Setup

10 Digital Isolator

There is a total of four crossing of the isolation boundary; the power transformer, the feedback and control

of the two synchronous MOSFETs. Usually an opto-coupler is used for isolation of the feedback signal

since this a relatively slow analog signal. Most opto-couplers are too slow to use for the synchronous

MOSFET gate drive. There are fast opto-couplers available but there is a big cost premium. Historically,

the most common approach has been to use gate drive transformers to provide isolation for the

synchronous gate drive signals. The transformers can be used to directly drive the MOSFET gates or the

transformers can be used to just isolate the control signal which is then applied to a gate driver IC on the

secondary side. Gate drive transformers have their challenges and limitations. Transformers cannot pass

DC. A given size transformer can only pass a finite voltage and time product across the isolation

boundary. After each on-time, the transformer needs to be reset, which imposes duty cycle limitations.

Further, during a sudden switch-off of the power converter, the DC restorer capacitor on the secondary of

the gate drive transformer does not have a quick discharge path. This will keep SR FET's turned on,

resulting in a non-monotonic decay of the output voltage.

These limitations can be addressed using a digital isolator. The digital isolators are CMOS devices that

use an RF coupler to transmit digital information across the isolation barrier. The isolation capability is up

to 2500 VRMS. In simple words, the digital isolators are similar to an opto-coupler. While, the opto-

couplers modulate light to transmit electrical signals, the digital isolators modulate an RF signal across a

semiconductor barrier. Furthermore, the digital isolators have lower propagation delay than the gate drive

transformers and do not suffer volt-second limitations.

11 Performance Characteristics

11.1 Turn-On Waveforms

A certain sequence of events occurs when applying power to the LM5035C evaluation board. The soft-

start capacitor values and other components allow for a minimal output voltage for a short time until the

feedback loop can stabilize without overshoot. Figure 1 shows the output voltage during a typical start-up

with a 48V input and a load of 5A. There is no overshoot during startup.

AN-2043 LM5035C Evaluation Board SNVA433A–March 2010–Revised May 2013

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