Gate Drive Transformers vs. Fully Integrated Isolators in Isolated DC-DC Power Converters

Figure 1: Half-Bridge Power Converter with Control Driven Synchronous Rectifiers
(Click on image to enlarge)

There are a total of four crossings of the isolation boundary; the power transformer, the feedback, and two synchronous rectifier MOSFET control signals. 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 isolation. 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 rectifier gate-drive signals. These 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. 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.

Gate-drive transformers have their challenges and limitations. Several vendors have recently started offering fully integrated alternatives to conventional gate-drive transformers. This article will review the challenges seen when using gate-drive transformers and describe some of the new alternatives now being offered.

Figure 2 shows the most basic transformer-isolated gate drive. The input is coupled through a dc blocking capacitor. Applying a 10V, 50% duty cycle drive signal will result in a DC of 5V across the blocking capacitor. The bias voltage value is: V(drive) × Duty Cycle.

Figure 2: Basic Transformer-Isolated Gate Drive

(Click on image to enlarge)

The final secondary-side drive amplitude in this example swings from +5V to -5V. The negative 'off potential' has advantages to provide very high noise immunity, but half of the peak 'on potential' is lost. This scheme is not very practical for duty cycles greater than 50%, as the peak amplitude continues to decrease with increasing duty cycle.

Care should be taken in applications where the duty cycle can change rapidly, such as transient response, which can result in erratic operation or damage. As the bias across the coupling capacitor is changing (due to a change in duty cycle) the capacitor can ring with the transformer magnetizing inductance.

This ringing can turn on the MOSFET at unintended time intervals. A larger capacitor value, gate resistance or slowing down the rate of change of duty cycle are options that can help. Selecting a value of capacitor too large can result in saturation of the transformer during transients.

Figure 3 shows another transformer-isolated gate drive, often called a DC-restored gate drive. The diode and the secondary-side capacitor restores the DC value of the gate drive and allows operation at larger duty cycles. This circuit can suffer from the same ringing and possible transformer saturation problems described in the basic approach.

Figure 3: Transformer-Isolated Gate Drive with DC Restore
(Click on image to enlarge)

There is an additional danger with this circuit during turn off (Reference 1). At turn off, the primary-side capacitor is connected directly across the primary for an indefinite time. The primary magnetizing current can build up, saturating the transformer. When the transformer saturates, the transformer secondary becomes a short, allowing the secondary-side capacitor to turn on the MOSFET, possibly damaging the power converter. Small-value coupling capacitors can help reduce this effect. A controller with a soft-stop feature, where upon turn off the duty cycle is gradually reduced rather than abruptly stopped, can also help (Reference 2).

Generally speaking, with careful design and evaluation, a transformer-isolated gate drive works fairly well for operation with duty cycles of 50% or less. For applications such as the power converter shown in Figure 1, the required duty cycle of the synchronous rectifiers can be quite high, well over 50%. For these high duty-cycle applications, transformer isolation will require the DC-restore technique which has potentially even more pitfalls and will require very careful design and evaluation.

Designers of high-performance isolated dc-dc power converters are always striving for higher efficiency and smaller size. A transformer-based, isolated gate drive is relatively large, requiring not only the transformer but also the associated reset components. Recently, several vendors have started to offer fully integrated, isolated, gate-drive solutions. These solutions employ several different isolation technologies ranging from micro-transformers, capacitor coupling with RF modulation and GMR (Giant Magneto-Resistive) sensors.

One popular family of isolation devices uses micro, chip-scale transformers to isolate digital signals across a ground isolation boundary (Reference 3). With each input transition, an edge is encoded such that two pulses indicate a rising edge and one pulse indicates a falling edge. The pulses are coupled through the micro-transformer and decoded on the secondary side. A refresh pulse is applied periodically from the primary to check for dc correctness. A watchdog timer is employed on the secondary side to look for the refresh pulse.

Another popular family of isolation devices uses high-frequency RF modulation to transmit digital information across a ground isolation boundary (Reference 4). On the primary side, a 700-MHz modulation signal is keyed 'on' or 'off' to indicate the presence of a 'one' or a 'zero' on the input. The field from the primary-side transmitter is detected on a secondary-side receiver. A demodulator decodes the absence or presence of RF to control the output state. The manufacture claims the RF on/off keying scheme provides best-in-class noise immunity, since the desired state information is constantly being sent and received at a very fast rate.

Yet another approach uses a technology called GMR or Giant Magneto-Resistive sensing (Reference 5). In this approach, when the primary-side input is a 'one', a DC current is gated onto a micro-coil and a concentrator which produces a focused magnetic field. On the secondary side is a GMR nanotechnology device made up of ferromagnetic alloys sandwiched around an ultrathin nonmagnetic conducting middle layer.

The sensors are arranged in a Wheatstone-bridge configuration. In the presence of a magnetic field, the resistance of the sensors changes, thereby changing the balance of the bridge. The bridge output is measured and conditioned with secondary-side circuits. The manufacture claims the lowest EMC noise signature of any high-speed digital isolator on the market today, because of the DC nature of the GMR sensors used.

All of these new, fully integrated isolators are very interesting. They promise the ability to transmit gate-drive information across the isolation boundary more reliability and in a smaller size than a classic transformer-based gate drive isolator. Power-converter applications can be challenging for any isolation scheme.

One area to pay careful attention is dV/dt susceptibility. The idea is to quickly slew the potential from one ground to the other ground to see if the differential output of the isolator maintains state, during and following the transient.

Electromagnetic susceptibility is another concern. The isolator must maintain the proper state while subjected to external fields. The temperature range of many of the new devices is limited to 85 degrees Celsius, which may be too low for some power-converter applications. Most of these new technologies require an independent 5V bias for both the primary side and the secondary side of the device.

This may require additional support components as compared to classic isolation transformers. The input to these new devices is generally configured for TTL thresholds, limited to a maximum of 5V. Some new controllers, such as National Semiconductor's LM5035C, have control outputs which swing from 0 to 5V in order to be directly compatible with the new technology isolators (Reference 6).

Recently there have been many new, promising isolator technologies introduced. The actual internal workings are very different among these new isolators; including micro-transformer pulse, RF keying and GMR. Before adopting any new technology a careful evaluation of the different technologies should be undertaken, the overall power-converter solution is only as good as the weakest component.

References

1. Ridley, Ray, "Gate Drive Design Tips" Power Systems Design Europe, Dec 2006
2. LM5035A Datasheet, National Semiconductor Corporation
3. Analog Devices Application Note AN-825 "Power Supply Considerations in iCoupler Isolation Products"
4. Silicon Labs Si8420 Datasheet
5. NVE Corporation web site "How GMR Works"
6. LM5035C Datasheet, National Semiconductor Corporation

About the Author
Robert (Bob) Bell is the applications engineering manager for National Semiconductor's design center located in Phoenix, Arizona. Products designed at the Phoenix design center include; integrated switching regulators, next generation PWM power controllers, gate drivers, hotswap and load-share controllers. He has been with National Semiconductor since September 2001. Prior to joining National, Bob designed power converters for military and space applications. Bob holds a Bachelor of Science degree in Electronic Engineering from Fairleigh Dickinson University, Teaneck N.J. To date, he has published 25 power-design articles, eight conference papers and three patents. In his spare time he enjoys; hiking, camping, tennis and travel.