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Benefits of integrated signal, power isolation

Posted: 13 Aug 2013 ?? ?Print Version ?Bookmark and Share

Keywords:industrial sensors? RTD? thermocouples? isolation? opto-couplers?

Two cross-coupled HVCMOS switches together with a centre-tap transformer provide the sustained oscillation, which is enabled or stopped by a tail switch controller by the feedback PWM signal. The Schottky diodes for efficient rectification at 120MHz and a modified type-II controller are implemented in die 2. For smooth power up, a soft-start circuit is implemented in die 1.

At power up, the soft start circuit provides PWM signals with slowly increasing duty to slowly charge the output voltage to a pre-set level slightly lower than the targeted output voltage, at which the feedback signal is enabled and it takes over the control of the tail switch from soft start output. This soft start sequence ensures minimum output overshoot at the power up, which is important for system reliability.

To transmit the data robustly across isolation barrier, differential encoding and decoding is used. The leading edge is encoded as a short pulse of positive polarity and the falling edge is encoded as a short pulse of the same duration but of negative polarity. The differential receiver would convert these pulses of different polarities into the output with the correct logic state.

To ensure the output is updated when the input has not changed state for a long time, periodic refresh pulses are also transmitted across the transformers, a positive pulse for logic high and a negative pulse for logic low. Differential receiver allows the common mode pickup during common mode transient events be rejected and large common mode transient immunity greater than 50kV/?S is achieved.

3.75kV, 1 minute, UL rated isolation is achieved through 32?m thick polyimide layers sandwiched in between the primary coils and the secondary coils. A cross-section for the isoPower transformers is shown in figure 3. Both the top winding and the bottom winding have 6?m thick Au for efficient power transfer. The polyimide is very good against HV transients, providing over 10kV surge capability. Besides high surge capability, the polyimide has also well-defined ageing behaviour.

Figure 3: Cross-section of isoPower transformers.

The time to failure is collected at various voltages from 1.2kV to 3kV, and it is plotted in figure 4.

Figure 4: High voltage lifetime for isoPower products.

Based on fit to a typical polyimide ageing model, L~eV-n, the life time is over 100 years at 400Vrms, good enough for many industrial applications.

Figure 5: isoPower transformer configuration.

Besides high voltage behaviour, EMC requirements such as radiated emissions are of concern for industrial applications. The isoPower transformers are closely coupled with only a few hundred?m in diameter, so radiated emissions from the transformers are very limited. The isoPower transformers are also implemented is s-type configuration as shown in figure 5.

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