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Easy way to crank a simulator

05 May 2014  | Matthias Ulmann

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Electronic engineers working in the automotive domain are inevitably going to face a 'Cranking Test Pulse'. These test pulses describe the drop of the battery voltage during cranking of the engine and all car manufacturers have their own standard for them. Since plenty of electronic circuits are attached to the battery, these are impacted by this event. In some applications like the navigation- or multimedia- system, an interruption of operation due to the drop of the input voltage is not wanted or even acceptable. In this case, mostly a boost convert is placed in front of the circuit to provide a stable input voltage for the electronics.

During the development process the functionality of this pre-booster must be tested to ensure a fast start-up and a clean and stable output voltage for the subsequent electronics like point-of-load converters. A typical solution for this kind of applications is Texas Instruments TPS43330 providing two synchronous buck converters and a boost. The battery voltage is connected directly to the boost and the two bucks are connected to the output of the boost.

As soon as the battery voltage drops below an adjustable threshold, the boost starts up and supplies the bucks with a constant voltage of 7V, 10V or 11V.

Plenty of manufacturer offer test system to simulate cranking pulses, but unfortunately they have also 'commercial' prices. To test automotive electronic systems up to 50 W input power with different standardised cranking pulses, the small and inexpensive cranking simulator shown in the following can be used.


Specification
Basically, a flexible programmable, arbitrary signal generator is needed, which covers an output voltage rang of 2 V to 15 V and a maximum output power of 50 W. These requirements can be divided into three areas.

For the power section, a buck converter is the right choice, as no galvanic isolation is needed and it generally achieves the highest regulation bandwidth of all non-isolated topologies. As the output voltage is in the range of 2 V to 15 V, an input voltage of 24 V DC is ideal which can be provided by a normal power supply found in every lab.

To increase the output voltage of the buck converter, just the duty cycle has to be increased and the direction of the current flow inside the inductor keeps the same. If the output voltage has to be decreased very fast, it is not enough just to reduce the duty cycle. Also the output capacitors have to be discharge to reach the new output voltage as fast as needed.

Figure 1 shows the power stage of a non-synchronous and a synchronous buck converter. The diode used in the non-synchronous topology allows a unidirectional current flow only. If the diode is replaced by a FET, the buck controller switches on this low-side FET continuously if the voltage on the output capacitance is higher than the new value set. Then the direction of the current flow in the inductor changes and the output capacitors are discharged by connecting them via the inductor to ground.


Figure 1: Non-synchronous and synchronous buck converter.


Of course, in applications like this where the duty cycle can be very low and the output current is high, a synchronous buck offers also much better efficiency than a non-synchronous approach where the forward voltage drop of the diode causes high losses.


Figure 2: Duty cycle on a buck converter.


To achieve a fast regulation of the output voltage, any 'Diode Emulation' or 'Power Safe Mode' of the controller must be disabled to keep the converter always in continuous conduction mode. This 'Forced PWM Mode' increases the losses at low load compared to discontinuous conduction mode, but plays no role for this application.

The TPS40170Vage mode buck controller from Texas Instruments fulfills all requirements and the high bandwidth (typ. 10MHz) of the integrated error amplifier enables a fast change of the output voltage.

The second area covers the variable and also fast change of the output voltage. Several approaches are possible to change the output voltage during operation, but probably the fastest one is know from powering DSPs (Digital Signal Processor). Dependent on the processor load, the core voltage is adjusted to increase the computing power or to reduce the losses. Usually this is done by a VID interface (Dynamic Voltage Identification) as shown in figure 3.


Figure 3: VID interface.


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