Synchronous vs. Nonsynchronous DC/DC Conversion

The diodes shown in the above image are key to operating the most common type of voltage regulator circuit you’ll find in a power converter. These provide a core function in power regulators: rectification. This action converts an oscillating (or switching) current into a DC current with some ripple, which can then be filtered and provided to load components in the system. Switching regulators that use a diode as the rectifying element are known as nonsynchronous converters.

Although the typical introductory circuit diagram and discussion of switching regulators focuses on nonsynchronous converters, these are not typically used in switching regulator ICs or in systems built with discrete components. The other type of switching regulator is a synchronous regulator, which uses a transistor as the rectifying element on the low side of the switching node. Aside from the use of a diode vs. transistor, how are these systems different in terms of operation? We’ll address this question in this article.

Two Types of Voltage Regulator Circuits

The image below shows two types of power regulator circuits: synchronous and nonsynchronous buck converters. Note that other topologies, such as buck-boost, boost, flyback, etc., can be constructed as synchronous converters simply by replacing any rectifying element in the system with a transistor.

Synchronous and nonsynchronous buck converter circuits.

When two MOSFETs are placed in a synchronous converter, they both need to be driven with a switching PWM pulse. The pulses sent to the high-side and low-side MOSFETs are alternating during each cycle in the converter (they are ON and OFF at alternating times). This will cause the time window for the driving pulse in the synchronous converter to match the diode’s forward bias window in the nonsynchronous converter. This means the same duty cycle relations found in a nonsynchronous converter can apply to a synchronous converter.

Synchronous vs. Nonsynchronous Operation

Clearly, these two types of circuits are very similar, so how do they differ in terms of their capabilities and operating characteristics? The table below highlights differences in a few important operational areas for each type of converter.

Switching in Synchronous Operation

The synchronous operation mode involves turning the low-side transistor ON at the moment the high-side transistor is turned OFF, and vice-versa. The system’s duty cycle, which will determine the step-down or step-up ratio, will be the high-side MOSFET duty cycle.

In synchronous operation, the points that matter for successful operation in nonsynchronous converters also apply here. However, switching between the two converters requires enforcing some dead time, otherwise there will be a momentary short circuit when the ON state switches from the high-side to the low-side. This occurs because the switching waveform does not switch between HIGH and LOW states instantaneously. Instead, the edge rates can slightly overlap. Typical waveforms seen in these converters are shown below.

Switching waveforms and inductor current example in a synchronous converter.

Resources For Simulating Switching Regulators

Any time you want to simulate a switching regulator in SPICE, the regulator must have correct models that accurately capture the electrical behavior of the components. For the diode in a nonsynchronous converter, the main point in the model that must be included in a simulation is the forward voltage of the diode, which will limit the efficiency of the converter to some maximum value. Although this is something you would find in a datasheet, you can usually adjust this value in your generic diode model in your SPICE simulator.

For the synchronous converter, the simulation will need to have complete MOSFET models that accurately capture three important quantities:

• Gate capacitance
• Source-drain capacitance
• ON-state resistance

The simulation will need to include these MOSFET characteristics in order to fully capture the effects of switching action on ringing at the switching node when the converter is in the continuous mode. This ringing would then be observed with a voltage probe at the switching node in a transient analysis simulation. Ultimately, the goal in these simulations is to identify the operating mode and the conversion efficiency, as well as any noise that would be conducted to the load.

When you’re ready to compare synchronous vs. nonsynchronous DC/DC converters for your system, you can design and simulate your circuits with the simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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