Designideas Edited by Bill Travis Oscillating Output Improves System Security Jorge Marcos and Ana Gómez, University of Vigo, Vigo, Spain

designideas Edited by Bill Travis Oscillating output improves system security Jorge Marcos and Ana Gómez, University of Vigo, Vigo, Spain any electronic-control systems have digital outputs Figure 1 Mthat use transistors. One STATIC method of improving the security in VARIABLE these outputs is to use an oscillating sig- DYNAMIC nal to represent a logic-high state instead VARIABLE of a fixed voltage level (Figure 1). This LEVEL 0 LEVEL 1 LEVEL 0 type of signal, a dynamic variable, can drive the circuit shown in Figure 2. This circuit connects to the output transistor of the electronic-control system. The dy- You can improve the reliability of an electronic-control output by using an oscillating, instead of namic-variable signal connects to the steady-state, signal to represent a logic-high state. output transistor, whose collector connects to a pulse transformer. If the system 24V cannot produce pulses because of a fault, the relay deactivates. If a fault exists in the drive circuitry, the output system stays 1N5347B ZENER 3 V ϭ10V off. This solution guarantees the securi- Z 1 4 ty function when one fault occurs. How- 1N4002 RELAY ever, it does not guarantee the detection 1N4002 5 16 2 1 ␮F 1 of all faults. However, you can use a 6 DPDT relay and connect one of 9 Figure 2 TRANSFORMER NO1 the contacts to one of the elec- SKPT25B3 COM1 13 tronic-control inputs; thus, you can con- 11 NC1 FUSE 8 firm the relay’s activation or deactivation NO2 4 in the control program. COM2 6 NC2 The circuit in Figure 2 performs effi- D ciently if the system can generate a fre- GATE_MOS quency greater than 1 kHz to excite the MOS G IRFD110 S Oscillating output improves system security................................................85 Polarity protector outperforms A “dynamic-variable” signal drives the output relay in this circuit. Schottky diodes ..............................................86 pulse transformer. Many electronic sys- square-wave signal of a frequency that Circuit offers improved tems that use an output transistor can suits commercial pulse transformers. active rectification ..........................................86 generate a dynamic variable. Many sys- This signal drives the MOSFET when the Build an adjustable high-frequency tems, such as PLCs (programmable-log- pulse detector receives pulses from the notch filter........................................................90 ic controllers), have long cycle times and, PLC. Because the pulse detector can fail, thus, cannot generate signals of adequate you must duplicate the external circuit Bootstrapping allows single-rail frequencies. In these cases, you can ob- for redundancy. In this way, the final re- op amp to provide 0V output ....................92 tain an appropriate signal using the low- sult is the same as that of Figure 2,ex- Filter allows comparison frequency dynamic variable from the cept that the safe output comprises two of noisy signals ..............................................94 PLC. To accomplish that task, you must relays. Publish your Design Idea in EDN. See the use an external oscillator and a pulse de- What’s Up section at www.edn.com. tector implemented with a monostable Is this the best Design Idea in this multivibrator. The oscillator produces a issue? Select at www.edn.com. www.edn.com February 6, 2003 | edn 85 designideas Polarity protector outperforms Schottky diodes Mike Hovenga, BAE Systems, Austin, TX he polarity-protection circuit in (often Schottky). The circuit incurs a MOSFET devices because of their low Figure 1 is a high-performance al- much lower voltage drop than even the on-resistance. For the transistors in this Tternative to the usual series diode best Schottky diode. The circuit uses design, the combined on-resistance is 0.013⍀. With a 10A load, the voltage drop is 0.13V at 25ЊC. Compare this fig- Q1 Q2 Figure 1 IRF7822 IRF7425 ure with forward-voltage drops of sever- al hundred millivolts for Schottky diodes + under the same conditions.You must use D1 p- and n-channel transistors in series be- IN4002 R2 R3 cause of their intrinsic diodes. A photo- 2.2M 2.2M voltaic isolator provides the appropriate 7 865 OUTPUT TO gate drive to the MOSFETs. The per- R1 100k PROTECTED INPUT 1.2k formance is even better at lower currents. CIRCUITRY IC1 PVI5033 You can replace the two discrete transistors by a single-package, complementa- D2 14V FOR ry-MOSFET pair, such as an IRF7389, 15.5V OVP 1 23 4 which has a combined on-resistance of ⍀ 0.108 . Resistors R2 and R3 are necessary to turn off the transistors when IC1 turns 10k off. R1 provides a nominal 12V input. Ϫ Is this the best Design Idea in this This polarity-protection circuit incurs lower forward-voltage drop than the best Schottky diodes. issue? Select at www.edn.com. Circuit offers improved active rectification Reza Moghimi, Analog Devices Inc, San Jose, CA ectifiers convert ac input is shown in red. Ͻ signals to dc. You can VCC If VIN 0V, the amplifier Rcombine a diode behaves as a comparator. Figure 1 VOUT1 VOUT2 and a load resistor to 3 Its negative input is at a + Vϩ create a half-wave rectifier, 1 higher potential than its VIN AD8591 provided that the amplitude 4 _ positive input, so its out- SD VϪ 1N4154 of the ac source is much larg- put,VOUT1, saturates to VEE. er than the forward drop of When the input again be- 5k the diode (typically 0.6V). VEE comes positive, the ampli- Unfortunately, you can’t use VCC fier has to recover from sat- this method to rectify signals uration and respond as that are smaller than a diode quickly as its slew rate and drop. For these applications, saturation recovery time active rectifiers using ampli- allow. This response takes fiers are available. The diode This circuit is a typical half-wave-rectifier configuration. some time, and the input is inside the feedback loop of may have changed by the Ͼ an amplifier (Figure 1). For VIN 0V, the conduct, the amplifier is in an open-loop time the amplifier is ready to respond to diode provides negative feedback, and the configuration, and VOUT2~0V. Figure 2 the positive input. The signals at VOUT1 output,V , follows the input (V ϭ shows the response of the circuit in Fig- (red) and V (green) clarify this point OUT2 Ͻ OUT2 OUT2 VIN). For VIN 0V, the diode does not ure 1. The output is shown in green; the (Figure 3).VOUT2 is the same waveform as 86 edn | February 6, 2003 www.edn.com designideas in Figure 2. Note the change in scale fac- tor. VOUT1 is a diode drop higher for positive inputs and saturates to VEE for negative inputs. The time delay in the response may result in a significant error in the output. For example, an amplifier that has a slew rate of 2.5V/␮sec and saturates to Ϫ2.5V takes at least 1 ␮sec to get ready to respond to positive in- Figure 2 These signals appear at the input (red) and the output (green) of the circuit in Figure 1. puts. During this time, the fast input has changed, so rectification starts at the wrong part of the input. One way to minimize this error is to use a high-slew- rate amplifier, but this solution comes at the expense of high power consumption. Another option is to use an inverting- amplifier configuration and two diodes, followed by a unity-gain inverting amplifier to obtain the noninverting rectification. This method appears in many textbooks. The circuit in Figure 4 repre- sents a one-stage, noninverting rectifier that improves the accuracy of the These signals are the waveforms at V (green) and V (red) in the circuit in Figure 3 OUT2 OUT1 rectification and reduces the pow- Figure 1. er consumption. In this circuit, an VCC AD8561 amplifier acts as a comparator. ϩ VOUT1 The AD8591 performs the rectification. 3 + V Ͼ 1 When VIN 0V, the output of the AD8591 4 _ AD8561 is high, and the AD8591 acts as SD VϪ 5k Ͻ VIN a follower. When VIN 0V, the output of V the AD8561 is low, and the AD8591 shuts CC VEE down. This shutdown puts the output of Vϩ 3 _ 1 the AD8591 into a high-impedance state, 8 7 so it remains at approximately 0V,rather AD8561 2 5 + VϪ than saturating to VEE as it did in the pre- 4 6 vious circuit.When VIN goes positive, the amplifier comes out of shutdown VEE Figure 4 and again follows the input. This This circuit greatly improves on the performance of the circuit in Figure 1. turn-on time (the time it takes to come out of shutdown) is much shorter than the saturation recovery time and slew- rate limiting that occurs in the previous circuit. Figure 5 shows the signals at the input (red) and output (green) of the improved rectifier circuit. Is this the best Design Idea in this issue? Select at www.edn.com. These signals appear at the input (red) and output (green) of the circuit in Figure 4. Figure 5 88 edn | February 6, 2003 www.edn.com designideas Build an adjustable high-frequency notch filter John Ambrose, Mixed Signal Integration Corp, San Jose, CA lthough you can obtain universal, bandpass filter, you can construct a notch quired 6V.This supply span matches well resistor-programmable switched-ca- filter (Figure 1). IC2, a TLC082 is a dual with IC1, an MSHFS6, with its 5V nomi- Apacitor filters that are configurable BiCMOS op amp, replacing the older nal operating voltage.

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