Even the simple buck converter can present very complex and changeable control characteristics when interacting with surrounding components and operating in different modes.  

Introduction

This column is going to be devoted to the complex and intriguing issues that continue to make switching power supply design a challenging and time-consuming task. I hope to show you the need for proper diligence and engineering in designing the power supply, a crucial and often-misunderstood part of electronic systems.

Outside of the power supply design community, there is also an ever-present feeling that power supply design is easy, and not worthy of time and attention. Nothing could be further from the truth, and numerous product recalls in recent year related to power and heat issues are a testament to this fact.

There is no doubt that power supply design is a mature industry. Standard circuits are used in around the world, with flyback circuits, buck converters, and forward converters dominating the field. This has held stable for many years now, and as a result, there are strong efforts from semiconductor companies to integrate functions into advanced chips with more and more capability inside the chip, and fewer parts on the.  

As the switching power supply functions become incorporated inside the chip, we often lose design flexibility, and access to crucial functions.  I’ll talk in a future column about the parts that are being integrated, and why I personally prefer access to many of them with discrete designs. In this column, we’ll focus on just one of the functions that can be overlooked, the feedback control loop.

It has always been a specialty skill within power supply design to work on control. Too often, in the latest round of IC designs, the pretense is made that this is no longer a function that needs attention, and integration of the feedback loop eliminates the chance for proper system design.

Integration of discrete power circuits has happened before in this industry. Thirty years ago, when we didn’t yet have switchers to deal with, the industry was dominated by linear regulators. Sophisticated designs were generated by experienced engineers to optimize parameters such as the minimum dropout voltage, transient response time, thermal characteristics, efficiency, etc.

Designers were highly knowledgeable in transistor characteristics, thermal design, and feedback analysis. Since all regulators use error amplifiers to precisely set the output voltage, feedback analysis and measurement was part of the design procedure for an optimized system.

Later, standard solutions arrived in the industry, leading to integration of the linear regulator. Today, few of us ever consider building their own linear regulator since it has all been effectively integrated. In the process, access to the feedback loop has been lost, but no-one seems to be concerned about this. Why not? Well, the integration of the linear regulator went fairly smoothly, and there are three reasons for this – 1) predictability, 2) consistency with line and load variations, and 3) low noise.

Feedback for the linear regulator is quite straightforward. The small-signal model is just that of a current source feeding a capacitor and load resistor. The only variation in the design of a control loop for the system is in the impedance of the output capacitor. Apart from this, the system is predictable, and can easily be simulated, and modeled, with modeling results agreeing closely with measuremkkents.

Figure 1: Once designed with discrete components, the three-terminal linear regulator is now almost always fully integrated. There is no longer any access to the control loop.

Once the linear regulator was integrated, there was no opportunity left to change the controller, except for changing the output capacitor. And the output capacitor was really the only uncontrolled component. However, there is little stress placed upon this part – it is there just to stabilize the system, and provide energy during load transient.

Once the linear regulator design is placed in the system, we can look at the characteristics of the regulator as the simple model shown in Figure 2. The input impedance of the linear regulator is a current source – regardless of changes in the input voltage, the current draw is fixed, equal to the output voltage divided by the load resistance. The output of the regulator model is just a voltage source.

Figure 2: As long as the linear regulator control loop is stable, the circuit model looks like a current source on the input (infinite impedance) and a voltage source on the output (very low impedance). Very little noise is introduced into the system.

Since the linear regulator does not generate any significant noise, we have no need to introduce any complex filtering on the system board. As a result, placement and integration of linear regulators on the board is a job that is almost trivial, and it does not require a power electronics designer to be involved (thermal issues notwithstanding – that’s often our responsibility, too.)

Does the future hold the same fate for the switching power supply? Will integration of the controller functions with power devices and auxiliary circuits render the switching power supply design a simple process?

Well, not quite so fast. As we’ll see below, even the simplest switching power supply has tremendous complexity of characteristics. 

Simple and effective gate drive schemes are presented, with some warnings about things that can go wrong.

Introduction

In this article, we return to basics of converter design – how to turn on and off the power FET in a modern dc-dc power supply. There have been many articles and application notes on this topic in the past, and the reader is encouraged to read these for more background information [1]. 

Direct Drive from the PWM Controller

Most modern control chips incorporate an output driver stage, usually consisting of a totem-pole arrangement of two transistors. This output can be used to directly drive the gate of a power FET, as shown in Fig 1.

Figure 1: The power FET is driven directly from the output of the PWM controller.

The direct connection can be used when the control circuit shares the same ground reference as the power circuit, and power levels are relatively low.

Data sheets show that several amps can be provided from the PWM controller output, more than enough to drive low-power devices. However, a FET input is a large capacitance, and it’s usually not a good idea to try and use the full available current. It can lead to increased EMI due to rapid turn on and turn-off, excessive reverse recovery loss in rectifiers, and noise issues inside the PWM controller itself. Clock jitter, and sporadic interruptions in the normal operations can occur. [2].

It’s a good idea to limit the current from the PWM controller with the network shown in Fig. 2. Two resistors are used– one to control the turn-on time, and one to control the turn-off time. A diode is used to separate the two functions, but can be omitted in some cases if the timings are less critical.

We usually turn on the FET slowly when running a low-power converter. Don’t be afraid to experiment with the value of the resistor R_on. I’ve used values as low as 1 ohm, and as high as 1 kohm in designs. My rule of design is to increase the resistor while monitoring the switching waveforms and power dissipation in the FET. If the temperature starts to rise significantly, cut the value of the resistor in half. For a DCM flyback, it is surprising how slowly you can turn on the device without significant switching loss.

Turn off needs to be faster to provide rapid shut down in case of overcurrent conditions. Experiment with different values, don’t just use the value that might be shown in an application note. For more information on how fast you have to control the FET, refer to [3].

Figure 2: Slow-down resistors are used to control the turn-on and turn-off time of the power FET. We usually turn the device off faster than it is turned on in order to achieve fast current protection.

Dedicated Gate Drivers

As power levels rise, you will find the values of the gate resistors need to decrease to minimize switching loss. For higher power circuits, it is common industry practice to use a high-current driver chip. This prevents interference with the PWM controller, and also allows better layout of the PCB. There are many good drivers on the market, or you can even build your own high-current totem-pole driver if you want to raise performance while reducing cost.

Figure 3: For larger power devices, and higher power switching, it’s a good idea to use separate gate drive circuits to switch the devices rapidly. Gate resistors are still used, but are not shown here. 

Transformer-Isolated Gate Drives

At higher power levels, we start to use topologies such as the two-switch forward converter, half-bridge converter, or full-bridge converter. All of these topologies require a floating switch to be driven.

There are silicon solutions to this problem. I would use these for low voltage applications, but not for off-line circuits. High-side integrated drivers remove too much control from the designer, and do not provide the same level of protection, isolation, immunity from transients, or common-mode noise rejection as a well-designed and implemented gate drive transformer.

Fig. 4 shows the most rugged way to achieve a floating drive. The output of the drive chip couples through a dc-blocking capacitor to a small transformer (usually a toroid for high performance.) The secondary is connected directly to the gate of the FET, and any slow-down resistors should be placed in the primary of the transformer. Note the use of zeners on the gate for transient protection. Catch diodes are needed at the output of the driver, and should not be omitted even if initial tests show there is no problem with the reactive current in the transformer.

Figure 4: The most rugged scheme for isolated drives uses a gate drive transformer as shown. Catch diodes are needed for reactive current drives, and a dc blocking capacitor prevent saturation of the transformer. The capacitor causes a level shift in the output drive voltage, and this varies with duty cycle.

The circuit of Fig. 4 provides a secondary gate waveform with a negative value when the FET is off. This greatly enhances common-mode noise immunity, crucial for a bridge circuit. However, the negative waveform also has the disadvantage of reducing the voltage applied when the switch is on. At short duty cycles, the positive pulse is largest. At a 50% duty cycle, half the available gate voltage is lost, and at large duty cycles, there may not be enough voltage to properly turn the FET on. The transformer-coupled circuit is most effectively used with duty cycles from 0-50%. Fortunately, this is exactly what is needed for the forward, full- and half-bridge converters.

Figure 5: The dc coupling capacitor can ring with the magnetizing inductance of the gate drive transformer, usually during start-up and transients. This ringing should be properly damped to ensure safe operation.

Notice in Fig. 5 that the dc coupling capacitor can give rise to a low-frequency ringing superimposed on the gate drive waveform. The usual solution to this is to use a large value of capacitor which lowers the Q of the ringing waveform. Make sure you test all transient conditions, especially start up when the capacitor is initially discharged.  

DC-Restorer Circuit – Watch Out!

Occasionally, you may run into a high-voltage circuit that needs an isolated gate drive close to 100%. In the past, the circuit of Fig. 6 has been recommended for this application.

Figure 6: This dc-restorer circuit has been suggested for many years for circuits that require an isolated drive in excess of 50% duty cycle. This circuit can very often lead to failure when the power supply is turned off, and it is not recommended.

A diode and capacitor on the secondary restore the dc value of the gate drive, and allow the gate to be driven to duty cycles of up to 90% or more.

However, there is a serious flaw in this circuit, and it is not recommended for use without very careful analysis. The circuit works well during steady-state operation (a gate load resistor is recommended), but when the PWM controller shuts off, the dc blocking capacitor is connected across the gate drive transformer for an indefinite period. This can lead to saturation of the transformer, as shown in Fig. 6b. When the transformer saturates, the secondary is a short circuit, and the secondary capacitor can turn on the FET.

The saturation can be avoided with a gapped core, and smaller value of capacitor, but this will increase the reactive current needed from the gate driver, and may produce other problems. 

Isolated Gate Drives for the Bridge Converters

The half- and full-bridge converters are isolated applications that need a very rugged drive scheme. During the switch off time, the opposite side of the bridge will turn on, impressing a high common-mode voltage to the off device. 

Figure 7: Two separate gate drive transformers are recommended for the half-bridge converter.   

Fig. 7 shows the recommended scheme for a half-bridge converter. Two gate drive transformers should be used, don’t try to get away with just one transformer with a tri-state operation that you may see in some application notes.

The full-bridge converter, shown in Fig. 8, also needs two gate-drive transformers. Dual secondaries on each transformer are used to drive the pair of FETs on the diagonally-opposite legs of the bridge. For both types of bridges, the gate circuit should be thoroughly tested during start up transients where the highest peak currents are seen, and the negative drive of the gate is the smallest.

Figure 8: The full bridge converter also uses two transformers for rugged design. Two FETs are driven from two separate secondaries of each gate drive transformer.

The phase-shifted bridge in Fig. 9 also has two gate drive transformers, but notice the different arrangement.  Each side of the bridge operates at a fixed 50% duty cycle, allowing the use of single gate drive transformer with dual secondaries of opposite polarities. This is one of the few circuits where the bipolar drive gate circuit can be used reliably. The only caveat is to be wary of the shut-down condition where ringing waveforms can turn on a device – there is no negative drive under this condition.

Figure 9: The phase-shifted full bridge uses a bi-directional transformer on each leg, Note the polarity of the secondaries. 

Summary

Gate drive circuits are a crucial part of design. Make sure you use the right scheme, and don’t just blindly copy an application note. Gate drive transformers add a level of ruggedness to your design that cannot be achieved with silicon solutions. If you are designing at high power levels, they are an essential element. While you need to think through all the components in a gate drive, be careful not to overcomplicate the design. Additional active elements to supposedly speed up the device switching do not usually offer improvements in overall performance, but they do introduce new potential failure mechanisms. Keep your gate circuits as simple as possible.

Additional Reading

• Join our LinkedIn group titled “Power Supply Design Center”. Noncommercial site with over 7000 helpful members with lots of theoretical and practical experience.
• For power supply hands-on training, please sign up for our workshops.
• “Design and Application Guide for High Speed MOSFET Gate Drive Circuits”, Lazslo Balogh, Texas Instruments Application Note.
• “Six Reasons for Power Supply Instability”, Ray Ridley
• “Power Supply Stress Testing”, Ray Ridley

Digital power is all the rage now, but don't expect it to shorten your design time, or to eliminate the need for complete optimization of the analog parts of the power supply.

Introduction

At the recent Applied Power Electronics Conference, digital power supplies were featured everywhere, in papers, seminars, and poster sessions. Applications were widespread, in VRMs, power factor correction circuits, inverters, and dc-dc converters.

It’s easy to start feeling overwhelmed with all this information on digital applications. And, in reading the material, to feel like you are perhaps missing the boat with your simple analog solutions. In this article, I’ll examine some of the issues and misconceptions about digital control.

Digital Control is New?

Figure 1 shows the block diagram of a digital controller. At the input side, an a/d converter samples an analog waveform of the power supply, and converts it to a digital value. The analog waveform may be the output voltage, or some pre-processed waveform where it has already been compared to a reference.

Sampling is done carefully, in an effort to avoid switching noise from affecting the results, and this simple sampling process can be quite complex, especially if there are multiple converters operating together in a system.

Figure 1: Block diagram of Digital Controller

The resulting digital signal then enters the processor of the digital controller, which sends the gate drive to the system. The driver circuitry may or may not be included in the controller, and this was discussed in the December 2006 issue of this magazine.

Much of the focus in recent years has been concentrated on solving the issues of the resolution of the digital output pulse. Numerous researchers have come up with solutions, including delay gates, to provide resolution beyond the clock frequency of the digital controller. This is important to avoid numerical oscillation.

Now let’s look at the so-called analog controller—yesterday’s technology if you listen to all the digital papers. Figure 2 shows a typical controller. The output voltage of a converter is processed with an analog amplifier, and discrete analog parts are used to compensate the feedback. The output of the error amplifier then is compared to a ramp with a comparator. The ramp also forms the clock of the converter.

Figure 2: Block diagram of “Analog” Controller

The ramp-reference circuit is nothing more than a simple A-D converter, directly generating a digital waveform, and setting the width of the pulse at the output of the controller. If you look at the input and output waveforms of Figure 1 and Figure 2, one thing should be immediately obvious: the analog controller that we have used for over 20 years in this industry is, in fact, a digital controller!

It has all the features required—and ADC, digital processing circuitry, and digital output. It is also an incredibly elegant solution to the digital controller problem. The clock frequency does not need to be any higher than the desired switching frequency of the power supply, and yet the output digital pulse has infinite resolution. This is something the new digital controllers are still struggling to achieve.

It is very important to recognize that the controllers we’ve always used are digital controllers. It stops us from worrying about whether we are using the latest in technology—analog or digital—when in fact they are both digital with different implementations. And it moves us forward to consider what the real difference is between the old and new, and whether you need it.

The core difference is that the new controllers are programmed with software, whereas the old controllers are hard wired, and not as flexible. As we’ll see in this article, the capability to reprogram may not be as important as claimed for your converter.