Interrupt Controller for DSP-based Control of Multi-Rail DC-DC Converters with Non-Integer Switching Frequency Ratio James Mooney, Simon Effler, Mark Halton, Hussain Mahdi University of Limerick 15/12/2010

Overview Introduction to DSP-based Control of Multi-Rail DC-DC Converter Systems Interrupt Management for Multiple Control Loops Modified Interrupt Controller Multi-rail DC-DC Converter Application Conclusions

DSP-based Control of Multi-Rail DC-DC Converter Systems Multiple DC-DC converters are compensated by a single DSP-based digital controller An interrupt signal triggers execution of a control algorithm when a new ADC sample is available

Interrupt-Triggered Control With Integer Multiple Frequency Ratios For multiple converters interrupt signals are interleaved so that each control loop’s interrupt service routine has a fixed time slot Constraining switching frequencies to integer multiples of each other can impact efficiency or performance of converters

Interrupt-Triggered Control With Non-Integer Multiple Frequency Ratios A delay in the calculation and updating of the duty cycle for at least one converter will occur if: An interrupt is triggered when a control algorithm is already being executed Multiple interrupt signals are triggered simultaneously The delay can vary each time an interrupt is triggered

Interrupt-Triggered Control With Non-Integer Multiple Frequency Ratios If duty cycle has not been calculated by beginning of next switching cycle, DPWM will apply duty cycle from previous cycle If load transient occurs: Duty cycle update delay will result in slower response in output voltage Instability could occur if delay occurs for a number of consecutive cycles

Maximum ADC Sample to Duty Cycle Update Delay To avoid problems with variable delay, fix delay at maximum for each iteration of each algorithm For a particular algorithm Maximum fixed delay is excessive and degrades performance of voltage regulator due to slower response to load transients

Modified Interrupt Controller Modified interrupt controller reduces T DMAX to acceptable value to obtain improved performance: All interrupts are automatically re-enabled after control algorithm has passed a certain stage of execution Allows interruption of one algorithm by another during pre-calculation stage, after duty cycle calculation and DPWM updating has been completed

Modified Interrupt Controller Improved interrupt scheme can be achieved by augmenting a conventional DSP’s interrupt controller with minimal additional hardware: Counter that determines when to re-enable interrupts Registers to store interrupt return addresses and duty-cycle calculation times for each algorithm in terms of number of instructions required

Multi-Rail DC-DC Converter Application FPGA Implementation Dual Datapath DSP core with modified interrupt controller Multi-rail switching mode power supply system 3 buck converters 12V – to – 1.5 V 500 & 495 kHz switching frequencies 3 rd order linear compensator applied to each converter 6 duty-cycle operations 6 pre-calculation operations

Interrupt Controller Operation - Comparison StandardModified

Interrupt Controller Operation - Standard 1)Int0 triggered 2)Int1 triggered 3)ISR0 executed 4)ISR1 executed

Interrupt Controller Operation - Comparison StandardModified

Interrupt Controller Operation - Modified 1)Int0 triggered 2)Int1 triggered 3)ISR0 started 4)ISR0 duty cycle calculation completed 5)ISR1 executed 6)Remainder of ISR0 executed

Performance Comparison Modified interrupt method has shorter T DMAX delay This facilitates the use of a wider bandwidth compensator Result: Improved performance in response to load step StandardModified

Conclusions Drawback of a standard DSP controlling multiple power converters is its limitation in dealing with switching frequencies with non-integer ratios ADC-sample to duty-cycle-update delay Existing DSPs have excessive delay Proposed method has a constant, reduced and hence more desirable delay Proposed interrupt controller performs significantly better in non- integer switching frequency applications Demonstrated using a three-rail power converter prototype

Thank you for your attention! Questions?

Thank you for your attention! Questions?

Backup Slides

Comparison with Standard Interrupt Method Manually enabling and disabling interrupts

Comparison with Standard Interrupt Method Separate interrupts for duty cycle calculation and pre- calculation code sections

Duty cycle updated early in switching cycle

Duty cycle updated just in time to be applied