DC/DC Converters Configuration and Control Methods

The DC/DC that is essential to the power supply of electronic devices, although called a DC/DC in general, the driver structure and the control method, etc. differ depending on the product.
Here, we describe the DC/DC driver structure, control method, and how much that difference impacts the characteristics, and so on.

1. Fundamental differences between the DC/DC converter and the DC/DC controller

The DC/DC converter and the DC/DC controller are distinguished according to whether a power MOSFET (driver transistor) is built-in or not.
This difference has a large impact on the characteristics of the respective ICs, on the simplicity of installment, and on the final cost.

DC/DC converter (internal FET type)

The DC/DC converter indicates a type in which the power MOSFET is built in to the IC in general.
Though, even for DC/DC converters, some products have both power MOSFETs built-in, as in the synchronous rectification method, while some products have only one power MOSFET built-in, as in the non-synchronous type.

DC/DC controller (external FET type)

The DC/DC controller is a DC/DC type in which the power MOSFET is not built-in, but is connected externally.
The choice of an external MOSFET enables handling of large currents and high efficiency, but increases the implementation area and the number of parts.
In addition, because it is necessary to choose peripheral parts, the knowledge to select parts is required and so the design difficulty tends to be greater than for the DC/DC converter.

	DC/DC capacitor (internal FET)	DC/DC controller (external FET)
Power MOSFET	Internal	External
Implementation area	Small	Comparatively large
Number of parts	Few	Many
Maximum output current	For low to medium current	For large current
Design simplicity	Comparatively simple	Complex (FET selection, layout, etc.)
Design freedom	Comparatively low	Comparatively high
EMI countermeasures	Comparatively simple	Complex (layout dependency)

2. Driver structure and the impact on characteristics

The DC/DC driver structure significantly impacts the efficiency and number of parts.

2.1 Non-synchronous type

This combines the Pch FET and SBD (Schottky diode).
The efficiency is inferior because of the power loss due to the forward direction voltage drop of the SBD. However, it has the merit that the DC/DC converter internal control circuit is simple to make.
The SBD is external, so the number of components increases compared to the synchronous rectification method.

2.2 Synchronous rectification method

This method synchronizes the MOSFETs and uses them instead of the SBD of the freewheeling diode.
Since there is no diode VF power loss, this method is able to achieve high efficiency both when the output current is high and when it is low.
In the case of a step-down DC/DC converter, a Pch is used for the High Side driver FET, while an Nch FET is used for the Low Side driver FET.

2.3 Bootstrap method

In the synchronous rectification method, this method uses an Nch FET for the driver for both the High Side and the Low Side.
This method has the merit that driver size can be reduced compared to the synchronous rectification method, because an Nch FET is used on the High Side.
Due to this, it is possible to achieve a lower on resistance with the same size, or increase compactness or lower costs if the performance is equivalent.

However, to drive the Nch FET gate on the High Side, a bootstrap capacitance is required since the voltage needs to be at least as high as the VIN voltage.

Driver structure	Main switch	Backfeeding element	Characteristics	Efficiency	Number of components
Non-synchronous type	Pch FET	SBD	Control circuit simple	Medium	Normal
Synchronous rectification method	Pch FET	Nch FET	High efficiency, large current handling, multiple controls required	High	Few
Bootstrap method	Nch FET	Nch FET	High efficiency, large current handling, multiple controls required	Extremely high	Normal

3. Control method

3.1 Control method (PWM or PFM)

The DC/DC control method controls the switching cycle and on time and can be a PWM control, PFM control, or PWM/PFM automatic switching control.
Among the differences between these methods, the light load efficiency, the noise characteristic, and the output ripple voltage have a significant impact.

Operation mode	Switching frequency	On duration width	Light load efficiency	Output ripple	Noise spectrum
PWM control	Fixed	Variable	Low	Small	Specific frequency
PFM control	Variable	Fixed	High	Large	Fluctuations depending on the load
**PFM/PWM automatic switching	Automatic switching in response to the load	Variable	High	Light load: large, Heavy load: small	Light load: fluctuations, Heavy load: specific frequency

3.2 Phase control method (voltage/current/COT control)

This is a phase control method which feeds back the kind of information into the IC to obtain a stable output voltage.
There are three main types of control method.

• Voltage control: directly feeds back the output voltage. The control loop is simple, but the transient response characteristics may deteriorate.
• Current control: feeds back the coil current, too, in addition to the output voltage. Compared to the voltage control, this control enables a high speed and stable response to load fluctuations and input voltage fluctuations.
• COT control: This method generates a fixed on time when the output voltage drops. The response to load fluctuations is extremely fast, and the compensation circuit is simple.

Control method	Feedback target	Transient response characteristics
Voltage control	Output voltage	Medium amount
Current control	Output voltage + inductor current	High speed
COT control	Output voltage (ripple)	Extremely high speed