Move energy conversion up a gear

Increased use of electronic systems in automotive applications is placing greater demands on the power systems that supply them. With pc-board space at a premium, the higher levels of load power necessitate system-level solutions with very high conversion efficiency and high power density.

The LM5010A, which is one of a family of hysteretic parts developed by National Semiconductor, can address these needs while still coping with the tough environment found in automotive systems. The design example system in this article achieves 90% full power efficiency whilst maintaining an IC temperature rise of less than 30°C above ambient.

Although trucks and buses may operate from 24V, most automotive applications still use a 12V battery system. When the engine starts in a 12V system, the high current drawn by the starter motor (>100A) can transiently reduce the battery voltage down to 6V. At the other extreme, voltage surges due to load dump can transiently increase the system voltage to around 70V. Whilst the design should be optimised to operate at highest efficiency at the nominal 12V level, the power-supply system should still be able to provide full power to the electrical systems over the full operating input voltage range.

Converter control

Most requirements for non-isolated step-down conversion can be met with the buck-converter topology (Fig 1).

The output voltage from this converter is approximately the input voltage multiplied by the duty cycle of the buck converter switch. Controlling the switching device's duty cycle can therefore regulate the converter's output voltage. Traditionally, this is achieved with running at a constant switching frequency and then varying the on-time of the device. However, for a buck converter to step 70V down to less than 5V, a duty cycle significantly below 10% is required.

In practice, this duty cycle combined with the trend to ever higher switching frequencies requires that the buck-converter switch is held on for a very short time, typically only a few hundred nanoseconds. This creates a significant problem for traditional high-frequency PWM controllers.

Another way to control the duty cycle is to run the switch with a constant fixed on-time and then vary the switching frequency to maintain regulation (pulse frequency modulation). This mode of operation is used with hysteretic converters.

As the equation shows, when a traditional hysteretic buck converter runs in continuous conduction mode with a constant on-time, the switching frequency FS will vary significantly as input voltage changes in order to maintain the output voltage at a constant level. In this equation, VD represents the forward conduction drop of the buck converter diode. For an automotive application running with an input voltage range of 6V to 70V, the variation in switching frequency will vary over a range of around 10:1.

As a result, the hysteretic operation simultaneously allows for a high step-down ratio and high switching frequency, both of which are essential for automotive solutions with restrictions on power-supply-unit (PSU) area. The only problem here is that the wide switching-frequency range can sometimes lead to unpredictable EMI and interference.

The LM5010A operates as a hysteretic controller and therefore yields the benefits mentioned above. The difference compared to traditional hysteretic operation is the on-time of the LM5010A switch is actually "programmed" by the input voltage. The LM5010A block diagram shows that the RON resistor sets the length of the on-timer by directly measuring the input voltage. As input voltage rises, the device on-time is reduced proportionately so that the switching frequency remains relatively constant over the full range of input voltages. The switching frequency will now only drop once the PSU enters discontinuous conduction mode under light load conditions. This transition point can be predicted through calculation.

The LM5010A operates as a digital on-off controller, and as such, doesn't need loop compensation components. This eliminates the delay normally associated with compensation, allowing the controller to respond to output load or input voltage changes in the next switching cycle. Also, since changes in input voltage change the device on-time almost instantly, rapid changes in input voltage have very little effect on output voltage. Thus, voltage fluctuation is virtually eliminated on the output during load dump conditions.

Designing with the LM5010A involves the designer deciding on what nominal switching frequency to run at. Operation at higher switching frequencies will minimise the size of the power-stage components, allowing for the use of small ceramic capacitors. The evaluation platform employed here runs at a nominal design frequency of 200kHz and can deliver +5Vdc at 1A from an input voltage of 6V to 75V. Figure 2 shows the full schematic for this converter.

Figure 3 shows the device's actual measured switching frequency as a function of load current at input voltages of 6V to 60V. Over most operating conditions of output load and input voltage, the device switching frequency is very close to the nominal 200kHz design value. A traditional hysteretic converter would demonstrate a much wider range of switching frequency variation with input voltage compared to the results given here.

At light loads (typically less than 10% of full load), the converter enters discontinuous conduction mode (DCM) and the switching frequency drops linearly with load. The point at which DCM mode begins can be predicted from this equation:

By choosing a higher nominal switching frequency or larger buck inductance, the converter can run down to a lower output current before there's a reduction in frequency (due to DCM). The lower operating frequency at light loads also helps to keep conversion efficiency high even down to very small loads. In the +5V evaluation board example used here, efficiency is around 90% with 12V input and greater than 82% even with 60V input.

Good thermal management is a necessity for a robust and reliable converter design. The LM5010A comes in two different package styles, both of which deliver a junction to case thermal impedance of just 40°C/W. At full output power of 1A, the LM5010A used in the design example demonstrated a temperature rise of just 30°C above ambient.

The new developments in constant on-time regulators have paved the way for integrated regulators to take the best features of hysteretic converters (e.g., ultra-fast transient response and simple implementation) and combine them with the predictable switching-frequency behaviour typically only found with PWM control schemes.

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