2004 35th Annual IEEE Power Electronics Specialists Conference
Yan Yin and Regan Zane
Colorado Power Electronics Center
University of Colorado at Boulder
Boulder, CO 80309-0425 yiny@colorado.edu, zane@colorado.edu
Abslrncf-This paper presents an approach for digital controller design and implementation in resonant inverters and electronic ballasts. The controller is designed around an inner phase loop for fast regulation of the resonant tank operating point and an outer current loop to control the output load current. The concept ofdirect digital phase control is reviewed, followed by a description of a complete dual-loop digital controller design. The controller is applied to an HID ballast control system with additional functions including lamp start- up and output over-voltage protection. The digital system is described with Verilog coding and can be synthesized and implemented automatically in standard digital logic. An
FPGA-based test bed for the digital controller is implemented for rapid prototyping. re-programmability, and realistic hardware realization. controller is not only cost effective, but also provides fine time resolution (down to sub nano-second), allowing accurate control even in the MHz switching frequency range.
In this paper, the design of a digital controller for an LCC resonant inverter is presented with inductor current phase regulation. The controller is successfully applied to a 400W
HID ballast controller. The control functions are described in Verilog and synthesized and implemented through automated CAD tools on a Xilinx FPGA for rapid prototyping and realistic hardware evaluation. The direct digital phase control concept is reviewed first in Section 11.
The key control blocks for the digital controller are discussed in Section 111, followed by an application to the
HID ballast with lamp start-up and over-voltage protection in Section IV. The FGPA implementation details are present in Section V along with experimental results. The conclusions are given in Section VI.
1. INTRODUCTION
High-frequency resonant inverters find a wide range of applications, including electronic ballasts, induction heating, and motor drives [I-41. The dc input power is generally converted to ac output through a switching network followed by a resonant tank. In most applications, a closed- loop controller is required to regulate the output waveforms for improved rejection of component tolerances, variations in environmental conditions, and variable power control.
Common approaches for regulating the output include control of duty cycle, switching frequency, or dc input voltage [5-71, where frequency control is one of the most popular control schemes due to its simplicity and wide dynamic range. Another promising control method is to regulate the relative phase angle between mid-point voltage and resonant inductor current or voltage. A direct digital approach to phase control was proposed in with frequency control, the phase control approach provides advantages such as self-tuning to resonance and less sensitivity to variations.
A digital controller implementation has many advantages over its analog counterpart, including low sensitivity to parameter and temperature variations, fewer external components and smaller size. It can also utilize the fast advances in digital CAD and silicon processing for full design automation, reprogrammability and low cost hardware implementation. By targeting standard sub- micron digital processes, the hardware-based digital
11. REVIEW OF DIGITAL PHASECONTROL
The direct digital phase control concept, strategy and hardware implementation were presented in [E]. The key concept and resulting control equation and state diagram for digital realization are briefly reviewed here.
A typical resonant LCC inverter is shown in Figure 1.
When it is operated above resonance, the resonant inductor dominates the resonant tank such that the input impedance of the inverter is inductive and the inductor current ir lags
(and hence the inductor voltage vL leads) the mid-point voltage v, from 0" to 90" as the switching frequency shifts away from resonance. The output power decreases as the phase angle varies from 0" to 90". Thus it is possible to control the output power by directly controlling the phase angle between the mid-point voltage and the inductor current or voltage. The switching frequency is thus
Figure I Typical resonant inverter configuration with LCC r a n a n t rank
This work IS sponsored by General Electric CO Global Research, through the
Colorado Power Electronics Center and
IS cofunded by the Department of
Energy's National Energy Technology Laboratory under Cwperatlve
Agreement DE-FC26-OZNT4 1252
0-7803-8399-0/04/$20.00 02004 IEEE. 1855
Aachen, Germany. 2004

2004 35th Annual IEEE Power Electronics Specialists Conference Aochen, Cermorry, 2004 r.
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Figure 2 Inductor current and mid-paint voltage in a half-bridge LCC monani inverter indirectly controlled through phase control. By controlling the inductor current phase angle to be greater than Oo, the system will be forced to operate above resonance, resulting in ZVS operation for appropriately designed resonant tanks.
If the resonant frequency shifts due to variations of the tank elements and load, the phase controller will self-tune to the resonant frequency on a near cycle-by-cycle basis and the operating point will not be affected.
The basic strategy to directly control the inductor current phase can
be illustrated by Figure 2. The essence of the
control is to time the period by detecting the inductor current zero crossing, then compute the required time delays from the zero crossing to determine when to turn on or off the high and low-side gates to achieve the desired phase.
Given a phase command, the time delay from the zero crossing of the inductor current to the falling edge of the mid-point voltage can be computed as
111. DESCRIPTION OF DIGITAL CONTROLLER
Figure 3 illustrates the system diagram for a digitally controlled LCC invtzrter with inductor current phase regulation. The functional diagram for the digital controller
is shown in Figure 4 .
The outer loop controls the output current (it can also he output voltage or power) sensed by a small resistor. The current is converted to a digital representation by an A D converter. The peakdetector block detects the peak value of the output current found for each switching cycle The detected peak current is then compared with a reference current and the error signal is sent to the compensator. The compensator generates the phase command to the inner phase loop to regulate the phase angle between the mid-point voltage and the inductor current. In the following, the design details for key functional blocks of this digital control system will be discussed.
A Phase controller
The basic concept of the phase control is reviewed in
Section II and the alesign implementation details were presented in [SI. The core of the phase controller is described in a synchronous state machine, where the state
diagram is illustrated in Figure 5 .
T,, is computed according to ( I ) . Th: phase command comes from the compensator, and the inductor current is sensed by a current transformer and converted to a square wave through a high- speed comparator. Thi: state machine runs at 200MHz with
5ns time resolution. where 0
< a,[n] 2 2"
-
1, m is the number of bits of the digital phase command, and a,[n] =
0 corresponds to 0" while al[n] = 2" - 1 corresponds to 90".
The control scheme based on Figure 2 is:
(a) Detect the zero crossing of the inductor current using a high-speed comparator;
(b) Record T, of the previous cycle, which is the time interval between two positive zero-crossing points;
(c) Compute T& according to (I);
(d) Wait for Td+, then turn off the high-side (HS) gate and turn on the low-side (LS) gate with a proper deadtime;
(e) Wait for half of C, on the HS gate and turn off the LS gate with a proper deadtime;
(f) Wait for the next zero crossing of the inductor voltage, then repeat this cycle.
Similar control scheme can also be applied to the inductor voltage phase control [SI. This control scheme can be easily realized using standard digital logic and implemented in a custom CMOS digital control IC or programmable logic. It can also be implemented in a microcontroller or DSP.
R
=
4 0 0
C, = 5nF comparator
Figure
3 System diagram for digitally eontrolled LCC inverter wilh inductor current phase control
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2004 35th Annual IEEE Power Electronics Specialists Conference
comparator
Aachen. Germany, 2004
Figure 4 Funaianal diagram for digital controller
B MD converter
The AID converter converts the sensed output current to an 8-bit digital representation. The full scale of the current is flOA and the LSB of the current represents about 78mA.
The A/D conversion is clocked at 32 times of the switching frequency and synchronized to the positive zero crossing of the inductor current.
C. Peak detection
The peak detector finds the peak current for each switching cycle. The finding process can be illustrated using
Figure
6. A temporary register is used to store the peak value. The final peak value for each switching cycle is latched by peak-/atch that is generated by the phase t
Figure 5 State diagram of digital phase conuoller
Figure 6 detector
I peak-latch controller at the positive zero-crossing of the inductor current. The temporary peak register is cleared at the same time to prepare for the next switching cycle comparison.
D. Compensator
The design of the compensator is according to the small- signal transfer function from the inductor phase to the output current as shown in Figure 7, which is derived using the small-signal model developed in [9] at steady-state operating point 9, = 73.8' and F,
= 14OHz. The model in
[9] was established to find the transfer function from switching frequency to the output current. With this model, the phase-to-output current transfer function is found indirectly as n
A A
G ( s ) =
- t f, j,
(2)
From the small-signal transfer function illustrated in Figure
7, it can be seen that it exhibits a single pole response in low-frequency range. Thus a simple integrator-type compensator is enough to achieve sufficient phase margin as well as very high low frequency gain. The digital compensator is designed according to the signal flow graph
shown in Figure 8 with the form of
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2004 M f h Annual IEEE Power Elecrronics Specialists Conference Anchen, Germany, 2004
-10
-20
-30 campensoror
4 0
.so
Figure 9 lamp conmller
MI
-10 ton 3w
Jk 1Ok JOk 5Ok be implemented internally for a standalone system. The
DCO runs at a frequency much higher than closed-loop system operating frequency range such that it does not interfere with normal operation.
Figure 7 Small-signal transfer function from inductor current phase to output current
Figure 8 Compensator design
I lp[k1[8:01
* v [ k l = v [ k - I l + / m l
(3) where e [ k ] =
/&I
- and p[k] is a 9-hit phase command.
The error signal generated hy the comparison is truncated to
4 hits to reduce processing requirements (smaller look-up tahle or multiplier).
With the gains of the A D and the digitalization of the phase taken into consideration, fl is selected as 0.25 to achieve the crossover frequency ahout 1 kHz. With this design, (3) can he easily implemented digitally by shilling the decimal point of the error signal two-bit left and adding it to the previous phase command. To improve the accuracy, the two decimal hits are always kept during the internal computation, which means p[k] is an 1 I-bit signal with two
LSBs being decimal hits. The final output is truncated to get a 9-hit phase output.
E. Digitally controlled oscillator (DCO)
The function of the digitally controlled oscillator (DCO) is to initiate the oscillation of the resonant tank, which is necessay for detection of the inductor current for phase regulation. When the system starts to oscillate, the closed- loop controller takes over the control of the system. The
DCO mode and closed-loop mode outputs are selected by a multiplexer as shown in Figure 4, which is controlled hy an external mode selection signal in our test bed and can also
IV. APPLICATION OF THE DIGITAL CONTROLLER TO HID
BALLAST
The digital controllsr described in Section 111 has been applied to control a complete HID ballast system. For this application, additional supervisory and control functions are required beyond the tiasic controller of Section 111. Here, two additional functions are described and implemented to demonstrate the flexibility and capability of the digital approach. First, a lamp start-up (ignition) controller is used to sweep the lamp voltage to ignite the lamp, followed by ignition detection and immediate transition to closed-loop control. Second, the output voltage is monitored for output over-voltage protection in order to prevent component failures and excessive !;tress.
A. Start-up controller.
The purpose ofthe start-up controller is to generate a high voltage to strike the iirc of the lamp. To incorporate the start-up function with the digital phase controller, the start- up controller sweeps the phase command from 90° toward resonance. Every 5ms (programmable), the phase decreases
ILSB (about 0.175O). Hefore the lamp ignites, the system is operating near open-circuit. During the phase sweep, the voltage across the lamp increases, until at some point the lamp ignites and the current flows through the arc. The digital controller keeps: monitoring the lamp current. When it detects a lamp current above a preset threshold, the phase sweep is stopped and .the closed-loop controller takes over the control and the system operates in closed-loop mode.
The diagram of the start-up controller is shown
in Figure 9.
If the ignition fails (no desired current detected) even if the phase has been swept
I:O a very low value (close to O"), the start-up controller will stop the sweep process and jump to
DCO mode. The phase command is set to 90" up controller will try to ignite the lamp again.
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2004 35th Annual IEEE Power Electronics Speciulists Conference Aachen. Germany, 2004
Figure 10 Syslem of FGPA-baEe digital HID c~nwllei
As the compensator is usually saturated during the lamp start-up process (because the lamp current is far from the reference current), the phase output of the closed-loop compensator (phase-cl[S:O] in Figure 9) is quite different from that of the start-up controller @hme-st[S:O] in Figure
9). To avoid the large transition during the switching from start-up mode to the closed-loop mode, the output of the compensator can be forced to he equal to the output of the start-up controller during the start-up process and the compensator works normally only after the lamp is ignited.
B. Over-voltage protection
The over-voltage protection module monitors the lamp voltage by a voltage divider. When over-voltage happens, it immediately shuts down the gate driver and switches back to
DCO mode, which runs at a much higher frequency and generates a very low output voltage. The system will not restart until the over-voltage is removed and the lamp cools down. v.
FGPA-IMPLEMENTATION HID BALLAST
CONTROLLER AND EXPERIMENTAL RESULTS
The digital controller discussed in Sections 111 and IV can be implemented in an ASIC with standard digital process.
For rapid prototyping and hardware evaluation, it is realized using a Xilinx Virtex II xc2v1000 FPGA. Figure IO illustrates the system diagram for the test bed. All functional blocks except the AiD converter are described in Verilog and synthesized and implemented automatically in the
FPGA, as shown inside the dashed box. The inductor current is sensed through a current transformer (CT). The AID converter is THS 1230 from Texas Instruments, which is a
12-bit AiD with only eight bits being -used for our application. An external switch is used to select DCO mode
(250KHz) or closed-loop mode. When the closed-loop mode is selected, the start-up controller initiates the phase-sweep until the lamp ignites, then the system runs at closed-loop mode. The over-voltage threshold is set to 3KV. The lamp voltage for the normal operation is about 200V and the ignition voltage is about IKV. The normal operating frequency for full power is I40KHz.
The steady-state operating waveforms are shown in
Figure 11 and the step response is shown in Figure 12. It can be seen that the system is stable. Figure 13 illustrates the start-up process with a 40R resistive load. The system first runs at DCO mode for IOms, then starts to sweep the phase.
When the current reaches the threshold that is set to be the same as the steady-state value, the system enters into the closed-loop operation mode smoothly.
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2004 351h Annual IEEE Power Electronics Specialisls Conference Aachen. Germany, 2004
(a) Lamp peak current
=
3A. lamp power = 350W
I
:,
\”
. .
.
I
. . .
.
.
. .
. * . . .
Figure I2 Step response (lamp peak current: 2.5A3 2.8A) (chl: lamp voltage: ch2 lamp curient, ch3: inductor current; voltage)
!
,.. . . . . .
4
!
L
... . i
(b) Lamp peak current = 2A, lamp power = 190W
Figure
I c h 3 inductor current; ch4: mid-point voltage) i
VI. CONCLUSIONS
This paper presents a digital controller design for resonant inverten and electronic ballasts with resonant inductor current phase regulation. The key control blocks are discussed that can be described with Verilog and implemented in standard digital logic with automatic synthesis, layout and route. The digital controller is successfully applied to a 400W HID ballast and implemented in an FGPA for rapid prototyping and realistic hardware implementation.
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