Cranking simulator made easy
Introduction
Electronic engineers working in the automotive area are sooner or later faced with 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 7 V, 10 V or 11 V.
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 standardized 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 TPS40170 voltage mode buck controller from Texas Instruments fulfills all requirements and the high bandwidth (typ. 10 MHz) 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.
The output voltage of a converter can be changed either by changing the reference voltage or the voltage, which is compared with the reference voltage. As the reference voltage is mostly fixed and not accessible on the controller, the second method has to be used.
Several additional resistors are placed in parallel to the low side resistor of the voltage divider, which can be switched on and off by small FETs.
In this circuit eight additional resistors and FETs are added which results in a resolution of 51 mV within the output voltage range of 2 V to 15 V.
To control the complete system, a microcontroller MSP430F2274 from Texas Instruments is utilized. Three different standard cranking pulses (DaimlerChrysler Engine Cranking Test Pulse DC-10615, Volkswagen Cold Start Test Pulse & Warm Start Test Pulse VW80000) are hard programmed in the MCU and they can be triggered either as single pulse or as a sequence of the same pulse with an adjustable delay in between. The MCU also handles the user interface
consisting of several push buttons and LEDs.
The firmware is programmed in C and care was taken for a modular and well commented structure. Even with little programming experience is shouldn’t be a problem to adapt the source code to your own needs, for example changing the shape of the pulses. The development environment ‘Code Composer Studio’ can be downloaded free of charge from ti.com and to program the microcontroller, TI’s inexpensive MSP430 LaunchPad can be used.
Some peripherals around the circuit like an active reverse polarity protection and trigger in- and outputs make the circuit more robust and easier to handle for the daily use in the lab.
Measurements
The most critical pulse regarding fast voltage change (Volkswagen Cold Start Test Pulse) is shown in Figure 4. Attached to the output is a constant load of 50 W and at the beginning of the test pulse the output voltage has to be reduced from 11 V to 3.2 V within less than one 1 ms.
Channel 2 (red) shows the output voltage and channel 1 (yellow) the output current. As the load is constant and the voltage decreases, the current has to rise in the same degree. The peak output current at this pulse is 26 A as it can be seen in Figure 5.
Figure 4 – Volkswagen Cold Start Test Pulse
Figure 5 – Change of the Output Voltage from 11 V to 3.2 V on a constant Load of 50 W
To show the performance of the system, a saw tooth waveforms was programmed. Channel 2 (red) in Figure 6 shows the output voltage rising linearly from 2 V to 15 V on a constant load of 50 W. Only the extreme voltage change from 2 V to 15 V is not quite perfect but still remarkable.
Figure 6 – Saw Tooth from 2 V to 15 V with a Period of 10 ms
Conclusion
The system shown here has been proven in daily lab use and completed all the tasks reliably. It is a small and inexpensive solution, but at the same time showing a great performance for testing car electronic like navigation and multimedia systems with respect to a cranking pulse on their input.
All information regarding this project like schematic, bill of material, layout and software are available on ti.com under the keyword PMP7233.
About the author
Matthias Ulmann was born in Ulm, Germany, in 1980. Ulmann was awarded a degree in electrical engineering from the University of Ulm in 2006. After working for several years in the field of motor control and solar inverters (specialized in IGBT-drivers), he joined TIs’ Analog Academy for a one year trainee program. Since 2010 he has worked in the EMEA Design Services Group as a Reference Design Engineer in Freising, Germany. His design activity includes isolated and non-isolated DC/DC converters for all application segments.
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